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Topic:Molecular biology

Molecular biology in horses involves the study of molecular processes and genetic mechanisms that underpin equine physiology and health. This field encompasses the analysis of DNA, RNA, proteins, and other biomolecules to understand gene expression, genetic variation, and cellular functions in horses. Techniques such as genomic sequencing, gene expression profiling, and molecular diagnostics are employed to explore topics like hereditary diseases, performance traits, and immune responses in equines. This page assembles peer-reviewed research studies and scholarly articles that investigate the molecular biology of horses, focusing on genetic research, molecular techniques, and their applications in equine science.
Incorporation of uracil into viral DNA correlates with reduced replication of EIAV in macrophages.
Virology    July 10, 1995   Volume 210, Issue 2 302-313 doi: 10.1006/viro.1995.1347
Steagall WK, Robek MD, Perry ST, Fuller FJ, Payne SL.The retrovirus equine infectious anemia virus (EIAV) encodes a dUTPase situated between reverse transcriptase and integrase. We have described the inability of EIAV with a 270-bp dUTPase deletion, delta DU EIAV, to replicate to wild-type (WT) levels in equine macrophages (D. S. Threadgill, W. K. Steagall, M. T. Flaherty, F. J. Fuller, S. T. Perry, K. E. Rushlow, S. F. J. LeGrice, and S. L. Payne, J. Virol. 67, 2592-2600, 1993). Here we describe the construction of a second dUTPase-deficient virus (DUD71E) containing a single amino acid substitution in dUTPase. delta DU and DUD71E replicate to ...
Monoclonal equine IgM and IgG immunoglobulins.
Veterinary immunology and immunopathology    July 1, 1995   Volume 47, Issue 1-2 1-12 doi: 10.1016/0165-2427(94)05381-2
Wagner B, Radbruch A, Richards C, Leibold W.In order to define equine immunoglobulins (Igs) and to produce monoclonal reference Igs we fused equine peripheral blood mononuclear cells with X63-Ag8.653 non Ig producing murine myeloma cells. A total of 29 equine Ig producing equi-murine heterohybridomas were obtained, of which ten expressed equine Ig for more than 3 months. One of these heterohybridoma lines produced monoclonal IgM, an equine isotype which has not been available in monoclonal form before. Four lines secreted equine IgG of two distinct Ig heavy chain types as assessed by the molecular weight (MW), while the remaining five l...
Complete primary sequence of equine cartilage link protein deduced from complementary DNA.
American journal of veterinary research    July 1, 1995   Volume 56, Issue 7 959-965 
Dudhia J, Platt D.Investigation of the structure of the equine articular cartilage link protein (LP) from individuals ranging in age from 1 to 15 years identified 3 distinct isoforms having molecular weights of 46,000, 43,000 and 41,000. The relative amounts of each of the 3 isoforms altered with age. The largest form did not change with age; however, amounts of the Mr 43,000 and 41,000 forms increased with increasing age. The results suggested that an accumulation, in the extracellular matrix of cartilage, of these 2 smaller products may have arisen from proteolytic cleavage. The complete amino acid sequence o...
Volume changes of the molten globule transitions of horse heart ferricytochrome c: a thermodynamic cycle.
Protein science : a publication of the Protein Society    July 1, 1995   Volume 4, Issue 7 1426-1429 doi: 10.1002/pro.5560040717
Foygel K, Spector S, Chatterjee S, Kahn PC.Volume changes among the unfolded (U), native (N), and molten globule (MG) conformations of horse heart ferricytochrome c have been measured. U to N (pH 2 to pH 7) was determined in the absence of added salt to be -136 +/- 5 mL/mol protein. U to MG (pH 2, no added salt to pH 2, 0.5 M KCl) yielded + 100 +/- 6 mL/mol. MG to N was broken into two steps, N to NClx at pH 7 by addition of buffered KCl to buffered protein lacking added salt (NClx = N interacting with an unknown number, X, of chloride ions), and MG to NClx by jumping MG at pH 2 in 0.5 M KCl to pH7 at the same salt concentration. The d...
Metal ion binding to apo, holo, and reconstituted horse spleen ferritin.
Journal of inorganic biochemistry    July 1, 1995   Volume 59, Issue 1 15-27 doi: 10.1016/0162-0134(94)00050-k
Pead S, Durrant E, Webb B, Larsen C, Heaton D, Johnson J, Watt GD.The binding of Cd2+, Zn2+, Cu2+, Ni2+, Co2+, Mn2+, and Mg2+ to apo, holo, reconstituted horse spleen ferritin (HoSF), and native holo HoSF with phosphate removed was measured by gel-exclusion chromatography. Three classes of strong binding interactions (Kd < 10(-7) M) with apo HoSF at pH 7.5 were found for the various M2+ studied: high stoichiometric binding (30-54 M2+/HoSF) for Cd2+, Zn2+, Cu2+, with two protons released per metal bound; intermediate binding (16 M2+/HoSF) for Ni2+ and Co2+, with one proton released per metal bound; and low levels of binding (2-12 M2+/HoSF) for Mn2+, Mg2+, and...
The DNA sequence of equine herpesvirus 2.
Journal of molecular biology    June 9, 1995   Volume 249, Issue 3 520-528 doi: 10.1006/jmbi.1995.0314
Telford EA, Watson MS, Aird HC, Perry J, Davison AJ.The complete DNA sequence of equine herpesvirus 2 (EHV-2) strain 86/67 was determined. The genome is 184,427 bp in size and has a base composition of 57.5% G + C. Unusually for a herpesvirus, about a third of the sequence distributed in several large blocks appears not to encode proteins. The 79 open reading frames that were identified as probably polypeptide-coding are predicted to encode 77 distinct proteins. Amino acid sequence comparisons confirmed that EHV-2 is a gamma-herpesvirus that is genetically collinear with herpesvirus saimiri (HVS; a gamma 2-herpesvirus) and Epstein-Barr virus (E...
A lectin histochemical study of gustatory (von Ebner’s) glands of the horse tongue.
Anatomia, histologia, embryologia    June 1, 1995   Volume 24, Issue 2 123-126 doi: 10.1111/j.1439-0264.1995.tb00022.x
Gargiulo AM, Pedini V, Ceccarelli P, Lorvik S.In the present work, gustatory glands (von Ebner's glands) of the horse tongue were examined by means of five peroxidase-conjugated lectins (PNA, DBA, SBA, UEA I, WGA), with and without prior sialidase digestion, in order to investigate the presence and distribution of carbohydrate residues in secretory cells and duct cells. The most intense staining of secretory cells was observed with PNA after pre-treatment with neuraminidase. This indicates that the terminal trisaccharide sequence sialic acid- (alpha 2-->3, 6) galactosyl (beta 1-->3) N-acetylgalactosamine is the most frequent oligosa...
Partial complementary deoxyribonucleic acid cloning of equine relaxin messenger ribonucleic acid, and its localization within the equine placenta.
Biology of reproduction    June 1, 1995   Volume 52, Issue 6 1307-1315 doi: 10.1095/biolreprod52.6.1307
Klonisch T, Ryan PL, Yamashiro S, Porter DG.To determine the site of relaxin gene expression in equine placentae, a set of degenerate oligonucleotide primers was made according to the published amino acid sequence of the A- and B-chain of equine relaxin (eRXN). Total cellular RNA (tcRNA) from equine placentae at about 120 and 300 days of pregnancy was subjected to reverse transcriptase-polymerase chain reaction (RT-PCR) with use of these primers. A single amplification product of approximately 430 bp was detected in each case by agarose gel electrophoresis. The PCR product was ligated into Bluescript plasmid and sequenced to confirm the...
At least four MHC class I genes are transcribed in the horse: phylogenetic analysis suggests an unusual evolutionary history for the MHC in this species.
European journal of immunogenetics : official journal of the British Society for Histocompatibility and Immunogenetics    June 1, 1995   Volume 22, Issue 3 249-260 doi: 10.1111/j.1744-313x.1995.tb00239.x
Ellis SA, Martin AJ, Holmes EC, Morrison WI.Nineteen horse MHC class I specificities have been serologically identified previously at a single locus (ELA-A), and two other specificities appear to be coded at other loci. Biochemical studies indicate that there are at least two expressed loci. In order to establish the number of transcribed horse MHC class I genes, we made a cDNA library from a heterozygous animal (ELA-A3/A7), and screened for positive clones using a bovine class I probe. More than 200 class I clones were isolated in this way, and so far seven unique full length sequences have been identified. All of the sequences are pre...
Extensive mtDNA diversity in horses revealed by PCR-SSCP analysis.
Animal genetics    June 1, 1995   Volume 26, Issue 3 193-196 doi: 10.1111/j.1365-2052.1995.tb03162.x
Marklund S, Chaudhary R, Marklund L, Sandberg K, Andersson L.The hypervariable D-loop region of mitochondrial DNA (mtDNA) was amplified with the polymerase chain reaction using total horse DNA samples. Analysis of single strand conformation polymorphism (SSCP) of denatured amplification products was carried out by native polyacrylamide (8%) gel electrophoresis followed by silver staining. As many as 15 distinct SSCP variants were revealed when screening a total of 78 maternally unrelated horses representing five different breeds. All breeds showed a high degree of polymorphism and the estimated probability (PImt) that two maternally unrelated individual...
Molecular cloning of cDNA for equine ovarian inhibin/activin beta A subunit.
The Journal of veterinary medical science    June 1, 1995   Volume 57, Issue 3 469-473 doi: 10.1292/jvms.57.469
Yoshida S, Yamanouchi K, Hasegawa T, Ikeda A, Suzuki M, Chang KT, Matsuyama S, Nishihara M, Takahashi M.cDNAs encoding equine inhibin/activin beta A subunit were isolated from an equine follicle cDNA library and characterized. Using primers based on the rat inhibin/activin beta A subunit cDNA sequence, a RT-PCR was performed to generate the probe for screening. Four positive clones were isolated. Analysis of the nucleotide sequence of these clones revealed that two pairs of identical clones were present, Eq-beta A-1 (0.9 kb) and Eq-beta A-2 (1.5 kb). Eq-beta A-2 clone contained a complete open reading frame encoding 426 amino acids. The deduced amino acid sequence of equine inhibin/activin beta ...
The cloning and expression of the alpha subunit of equine glycoprotein hormones.
Biochemical Society transactions    May 1, 1995   Volume 23, Issue 2 347S doi: 10.1042/bst023347s
O'Brien M, Headon DR.No abstract available
Sodium channel inactivation is impaired in equine hyperkalemic periodic paralysis.
Journal of neurophysiology    May 1, 1995   Volume 73, Issue 5 1892-1899 doi: 10.1152/jn.1995.73.5.1892
Cannon SC, Hayward LJ, Beech J, Brown RH.1. Equine hyperkalemic periodic paralysis (E-HPP) is a dominantly inherited disorder of muscle that causes recurrent episodes of stiffness (myotonia) and weakness in association with elevated serum K+. Affected horses carry a mutant allele of the skeletal muscle isoform of the Na channel alpha-subunit. To understand how this mutation may cause the disease phenotype, the functional defect in Na channel behavior was defined physiologically by recording unitary currents from cell-attached patches on normal and affected equine myotubes. 2. The presence of the mutation was confirmed in our cell lin...
Replication in vitro and in vivo of an equine infectious anemia virus mutant deficient in dUTPase activity.
Journal of virology    May 1, 1995   Volume 69, Issue 5 2881-2888 doi: 10.1128/JVI.69.5.2881-2888.1995
Lichtenstein DL, Rushlow KE, Cook RF, Raabe ML, Swardson CJ, Kociba GJ, Issel CJ, Montelaro RC.As an important enzyme in DNA synthesis, dUTPase is present in a wide variety of organisms and viruses and has been identified as a component of the equine infectious anemia virus (EIAV) pol gene. The role of EIAV dUTPase, designated DU, in virus replication in vitro and in vivo was investigated with a recently described infectious molecular clone of EIAV. A deletion mutant that was deficient in dUTPase activity was constructed, and its replication kinetics was examined in fetal equine kidney (FEK) cells and primary equine bone marrow macrophage (EBMM) cells. In FEK cells, which are permissive...
Regulatory function of the equine herpesvirus 1 ICP27 gene product.
Journal of virology    May 1, 1995   Volume 69, Issue 5 2786-2793 doi: 10.1128/JVI.69.5.2786-2793.1995
Zhao Y, Holden VR, Smith RH, O'Callaghan DJ.The UL3 protein of equine herpesvirus 1 (EHV-1) KyA strain is a homolog of the ICP27 alpha regulatory protein of herpes simplex virus type 1 (HSV-1) and the ORF 4 protein of varicella-zoster virus. To characterize the regulatory function of the UL3 gene product, a UL3 gene expression vector (pSVUL3) and a vector expressing a truncated version of the UL3 gene (pSVUL3P) were generated. These effector plasmids, in combination with an EHV-1 immediate-early (IE) gene expression vector (pSVIE) and chimeric EHV-1 promoter-chloramphenicol acetyltransferase (CAT) reporter constructs, were used in trans...
Interaction of beta-lactoglobulin with retinol and fatty acids and its role as a possible biological function for this protein: a review.
Journal of dairy science    May 1, 1995   Volume 78, Issue 5 978-988 doi: 10.3168/jds.S0022-0302(95)76713-3
Pérez MD, Calvo M.beta-Lactoglobulin is the major whey protein in the milk of ruminants and some nonruminants, such as pigs and horses. Although beta-lactoglobulin was first isolated 60 yr ago, no function has been definitely ascribed to beta-lactoglobulin. Recent x-ray crystallographic studies have advanced knowledge of the structure of beta-lactoglobulin, which is homologous with that of retinol-binding protein and lipocalycins; the function of these proteins seems to be participation in the transport of small hydrophobic substances. By analogy, this protein has been suggested as having a role as a transporte...
A novel Ca(2+)-dependent step in exocytosis subsequent to vesicle fusion.
FEBS letters    April 24, 1995   Volume 363, Issue 3 217-220 doi: 10.1016/0014-5793(95)00318-4
Hartmann J, Lindau M.Exocytosis begins with formation of a small fusion pore which then expands allowing rapid release of granular contents. We studied the influence of cytoplasmic free Ca2+ ([Ca2+]i) on the conductance of the initial pore and on the dynamics of subsequent expansion in horse eosinophils using the patch clamp technique. The mean initial conductance is approximately 200 pS independent of [Ca2+]i. This value is close to that previously found in beige mouse mast cells. The pore subsequently expands by 18 nS/s at [Ca2+]i < 10 nM, by 40 nS/s at [Ca2+]i = 1.5 microM and by 90 nS/s at [Ca2+]i = 10 micr...
Substitutions of isoleucine residues at the adenine binding site activate horse liver alcohol dehydrogenase.
Biochemistry    April 11, 1995   Volume 34, Issue 14 4709-4713 doi: 10.1021/bi00014a027
Fan F, Plapp BV.The contributions of isoleucine residues 224 and 269 of horse liver alcohol dehydrogenase to binding of the adenine moiety of NAD and to catalysis were studied by replacing Ile-224 with glycine (I224G) and Ile-269 with serine (I269S). The kinetic mechanisms of wild-type and both mutated liver enzymes were ordered. Affinities for several adenosine derivatives were decreased 5-50-fold by both substitutions. The I269S mutation differentially destabilized binding of the complete coenzyme, as affinities for NAD+ and NADH were decreased about 60-fold with the I224G enzyme and 350-fold for the I269S ...
Equine parentage testing by microsatellite locus at chromosome 1q2.1.
Animal genetics    April 1, 1995   Volume 26, Issue 2 123-124 doi: 10.1111/j.1365-2052.1995.tb02647.x
Sakagami M, Tozaki T, Mashima S, Hirota K, Mukoyama H.No abstract available
Permeation of small molecules into the cavity of ferritin as revealed by proton nuclear magnetic resonance relaxation.
The Biochemical journal    April 1, 1995   Volume 307 ( Pt 1), Issue Pt 1 253-256 doi: 10.1042/bj3070253
Yang D, Nagayama K.The NMR relaxation technique was used to investigate the permeation of molecules into the cavity of ferritin. Spin-lattice relaxation times in the rotating frame of various probe molecules were measured for solutions of recombinant horse L-apoferritin without iron and horse spleen apoferritin with very small amounts of ferric ions. The results show that molecules larger than the size of the ferritin channels can pass through the channels into the ferritin interior, and that the maximum size of molecules for the permeation is smaller than maltotriose.
Diversity of 16S rRNA genes of new Ehrlichia strains isolated from horses with clinical signs of Potomac horse fever.
International journal of systematic bacteriology    April 1, 1995   Volume 45, Issue 2 315-318 doi: 10.1099/00207713-45-2-315
Wen B, Rikihisa Y, Fuerst PA, Chaichanasiriwithaya W.Ehrlichia risticii is the causative agent of Potomac horse fever. Variations among the major antigens of different local E. risticii strains have been detected previously. To further assess genetic variability in this species or species complex, the sequences of the 16S rRNA genes of several isolates obtained from sick horses diagnosed as having Potomac horse fever were determined. The sequences of six isolates obtained from Ohio and three isolates obtained from Kentucky were amplified by PCR. Three groups of sequences were identified. The sequences of five of the Ohio isolates were identical ...
Synthesis and processing of equine herpesvirus 1 glycoprotein D.
Virology    April 1, 1995   Volume 208, Issue 1 9-18 doi: 10.1006/viro.1995.1124
Flowers CC, Flowers SP, Jennings SR, O'Callaghan DJ.Previous studies (C. C. Flowers and D. J. O'Callaghan, 1992, Virology 190, 307-315) employed peptide-specific antibodies to identify the product of the glycoprotein D (gD) gene of equine herpesvirus 1 strain Kentucky A (KyA). gD polypeptides of 55 and 58 kDa were detected in EHV-1-infected L-M cells, and the 58-kDa protein was observed in the membrane fraction of EHV-1 virions. In this report, the kinetics of synthesis and processing of gD polypeptides are described. One-hour pulse-labeling of EHV-1-infected L-M cells revealed that gD proteins are first detected at 6 hr after infection and tha...
Taxon-specific probes for the cellulolytic genus Fibrobacter reveal abundant and novel equine-associated populations.
Applied and environmental microbiology    April 1, 1995   Volume 61, Issue 4 1348-1351 doi: 10.1128/aem.61.4.1348-1351.1995
Lin C, Stahl DA.A total of six 16S rRNA targeted oligonucleotide probes were used to quantify Fibrobacter abundance and diversity in the gastrointestinal contents of a pony. Approximately 12% of the total 16S rRNA extracted from cecal contents hybridized with a Fibrobacter genus-specific probe and a Fibrobacter succinogenes species-specific probe. However, no significant hybridization was observed with a probe for the species. Fibrobacter intestinalis or with three probes for F. succinogenes subspecies. This suggested the presence of a previously undescribed population of F. succinogenes-like organisms. Novel...
Cloning and sequence analysis of a protective M-like protein gene from Streptococcus equi subsp. zooepidemicus.
Infection and immunity    April 1, 1995   Volume 63, Issue 4 1440-1445 doi: 10.1128/iai.63.4.1440-1445.1995
Timoney JF, Walker J, Zhou M, Ding J.Streptococcus equi subsp. zooepidemicus, a Lancefield group C streptococcus, is a frequently isolated opportunist pathogen from a variety of animal hosts, including the horse. Previous studies have indicated that equine strains carry antigens with characteristics of the antiphagocytic M proteins on the Lancefield groups A and G streptococci. We have cloned a protective M-like protein gene (SzPW60) of an equine strain of S. equi subsp. zooepidemicus W60 and determined its sequence. This gene encodes a protein with a molecular weight of 40,123 which protects mice against subsp. zooepidemicus but...
Delineating minimal protein domains and promoter elements for transcriptional activation by lentivirus Tat proteins.
Journal of virology    April 1, 1995   Volume 69, Issue 4 2605-2610 doi: 10.1128/JVI.69.4.2605-2610.1995
Southgate CD, Green MR.Lentivirus Tat proteins comprise a novel class of RNA-binding transcriptional activators that are essential for viral replication. In this study, we performed a series of protein fusion experiments to delineate the minimal protein domains and promoter elements required for Tat action. We show that a 15-amino-acid region of equine infectious anemia virus (EIAV) Tat protein, when fused to the GAL4 or LexA DNA binding domain, can activate transcription in appropriate promoter contexts. In the natural human immunodeficiency virus type 1 long terminal repeat, activation by Tat is dependent on multi...
Differences in a ribosomal DNA sequence of Strongylus species allows identification of single eggs.
International journal for parasitology    March 1, 1995   Volume 25, Issue 3 359-365 doi: 10.1016/0020-7519(94)00116-6
Campbell AJ, Gasser RB, Chilton NB.In the current study, molecular techniques were evaluated for the species identification of individual strongyle eggs. Adult worms of Strongylus edentatus, S. equinus and S. vulgaris were collected at necropsy from horses from Australia and the U.S.A. Genomic DNA was isolated and a ribosomal transcribed spacer (ITS-2) amplified and sequenced using polymerase chain reaction (PCR) techniques. The length of the ITS-2 sequence of S. edentatus, S. equinus and S. vulgaris ranged between 217 and 235 nucleotides. Extensive sequence analysis demonstrated a low degree (0-0.9%) of intraspecific variation...
Development of PCR assays to detect genetic variation amongst equine herpesvirus-1 isolates as an aid to epidemiological investigation.
Journal of virological methods    March 1, 1995   Volume 52, Issue 1-2 183-194 doi: 10.1016/0166-0934(94)00162-a
McCann SH, Mumford JA, Binns MM.A search for variable restriction sites has been carried out for equine herpesvirus-1 (EHV-1) in an attempt to develop markers which can be used to group epidemiologically related viruses into groups, and to learn more about the dynamics of EHV-1 disease. Crude viral DNA extracts of EHV-1, prepared by Hirt extraction, were digested with AluI, HaeIII, or RsaI, and Southern blotted following electrophoresis. DNA fingerprints, produced by probing the Southern blots with the EHV-1 EcoR1-I fragment, separated 56 isolates into 16 groups. The variable sites within the EcoR1-I fragment were mapped app...
Accumulation of chromotrope 2R positive cells in equine endometrium during early pregnancy and expression of transforming growth factor-beta 2 (TGF-beta 2).
Journal of reproduction and fertility    March 1, 1995   Volume 103, Issue 2 339-347 doi: 10.1530/jrf.0.1030339
Lea RG, Stewart F, Allen WR, Ohno I, Clark DA.Endometrial tissue from the gravid uterine horn of pregnant mares was examined by northern analysis and in situ hybridization for mRNA that hybridized to cDNA and RNA probes generated from a mouse TGF-beta 2 1.2 kb cDNA clone. The mouse cDNA probe hybridized to characteristic TGF-beta 2 mRNA transcripts on a northern blot of total RNA isolated from horse endometrium collected at day 45 of gestation. Two major 4.0 and 3.5 kb transcripts and possibly a minor 1.6 kb transcript were observed, consistent with specific hybridization to equine TGF-beta 2 mRNA. By in situ hybridization, riboprobes tra...
Identification of the causative agent of granulocytic ehrlichiosis in Swedish dogs and horses by direct solid phase sequencing of PCR products from the 16S rRNA gene.
Research in veterinary science    March 1, 1995   Volume 58, Issue 2 109-112 doi: 10.1016/0034-5288(95)90061-6
Johansson KE, Pettersson B, Uhlén M, Gunnarsson A, Malmqvist M, Olsson E.Seven Swedish isolates of Ehrlichia species from the blood of four dogs and three horses with clinical granulocytic ehrlichiosis, were identified by direct solid phase DNA sequencing of polymerase chain reaction (PCR) products from the 16S rRNA gene. The amplified DNA fragments were produced with primers complementary to the universal regions, U1, U2, U5 and U8 of the 16S rRNA molecule. Identical sequences were obtained from all seven isolates. This nucleotide sequence was similar to the sequences deposited in GenBank for Ehrlichia phagocytophila and E equi. The sequence of the Swedish ehrlich...
Enhanced sensitivity to neutralizing antibodies in a variant of equine infectious anemia virus is linked to amino acid substitutions in the surface unit envelope glycoprotein.
Journal of virology    March 1, 1995   Volume 69, Issue 3 1493-1499 doi: 10.1128/JVI.69.3.1493-1499.1995
Cook RF, Berger SL, Rushlow KE, McManus JM, Cook SJ, Harrold S, Raabe ML, Montelaro RC, Issel CJ.Serial passage of the prototype (PR) cell-adapted Wyoming strain of equine infectious anemia virus (EIAV) in fetal donkey dermal (FDD) rather than fetal horse (designated fetal equine kidney [FEK]) cell cultures resulted in the generation of a variant virus strain which produced accelerated cytopathic effects in FDD cells and was 100- to 1,000-fold more sensitive to neutralizing antibodies than its parent. This neutralization-sensitive variant was designated the FDD strain. Although there were differences in glycosylation between the PR and FDD strains, passage of the FDD virus in FEK cells di...