High‐frequency phase and antigenic variation of homologous lipoprotein haemagglutinins has been seen in both the major avian mycoplasma pathogens, Mycoplasma synoviae and Mycoplasma gallisepticum. The expression and, hence, antigenic variation of the pMGA gene family (encoding these lipoproteins in M. gallisepticum) is controlled by variation in the length of a trinucleotide repeat motif 5′ to the promoter of each gene. However, such a mechanism was not detected in preliminary observations on M. synoviae. Thus, the basis for control of variation in the vlhA gene family (which encodes the homologous haemagglutinin in M. synoviae) was investigated to enable comparison with its homologue in M. gallisepticum and with other lipoprotein gene families in mycoplasmas. The start point of transcription was identified 119 bp upstream of the initiation codon, but features associated with control of transcription in other mycoplasma lipoprotein genes were not seen. Comparison of three copies of vlhA revealed considerable sequence divergence at the 3′ end of the gene, but conservation of the 5′ end. Southern blot analysis of M. synoviae genomic DNA revealed that the promoter region and part of the conserved 5′ coding sequence occurred as a single copy, whereas the remainder of the coding sequence occurred as multiple copies. A 9.7 kb fragment of the genome was found to contain eight tandemly repeated regions partially homologous to vlhA, all lacking the putative promoter region and the single‐copy 5′ end of vlhA, but extending over one of four distinct overlapping regions of the 3′ coding sequence. Examination of sequential clones of M. synoviae established that unidirectional recombination occurs between the pseudogenes and the expressed vlhA, with duplication of pseudogene sequence and loss of the corresponding region previously seen in the expressed gene. Expression of the 5′ end of two variants of the vlhA gene showed that they differed in their reaction with monoclonal antibodies specific for this region. These data suggest that the control of vlhA antigenic variation in M. synoviae is achieved by multiple gene conversion events using a repertoire of coding sequences to generate a chimeric expressed gene, with the greatest potential for variation generated in the region encoding the haemagglutinin. Thus, completely distinct mechanisms have been adopted to control antigenic variation in homologous gene families.

The genome of the avian pathogen Mycoplasma galhsepticum contains a number of related genes for putative adhesion molecules @MGA). Cloning and sequence analysis of several pMGA genes suggested that all of them might be transcriptionally and translationally functional. Analysis of the gene sequence encoding the sole pMGA variant expressed in vitro in the S6 strain (pMGAl.1) revealed no unambiguous feature that could account for its unique expression. It is estimated that the pMGA gene family may contain up to 50 members, and its possible role is discussed herein.

A prominent feature of disease induced by Mycoplasma gallisepticum is a lymphoproliferative response in the respiratory tract. Although this is also seen in other mycoplasma infections, including Mycoplasma pneumoniae, the phenotype of the lymphocytes infiltrating the respiratory tract has not been determined. In this study, the numbers and distribution of lymphocytes in the tracheas of chickens infected with a virulent strain of M. gallisepticum were examined. Three groups of chickens were experimentally infected with M. gallisepticum and three unchallenged groups were used as controls. One infected and one control group were culled at 1, 2 and 3 weeks post infection.

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Mycoplasma synoviae has two major membrane antigens, MSPA and MSPB, both of which are phase variable and which may be coordinately involved in adhesion of the organism to erythrocytes. A single gene (vlhA) from M. synoviae was characterized, and polypeptides were expressed from nonoverlapping 5′ and 3′ regions in Escherichia coli. The expression product of the vlhA 5′ region reacted with specific reagents against MSPB, while that of the 3′ region reacted with specific reagents against MSPA. Analysis of the predicted amino acid sequence showed a characteristic signal peptidase II cleavage site, and the presence of the acylation site was confirmed by identification of a lipid-associated membrane protein, similar in molecular mass to MSPB, in [3H]palmitate-labelled membrane proteins. Further sequence analysis of the vlhA gene revealed a high identity with the Mycoplasma gallisepticum pMGA1.7 gene, a member of a large translated family. The vlhA gene was shown to hybridize to multiple restriction fragments of the M. synoviae genome, suggesting that it was also a member of a multigene family. These findings indicate that coordinate phase variation of the two major surface antigens of M. synoviaeWVU may be due to their expression from the same gene and that homologous gene families encode the major hemagglutinins of two phylogenetically distinct mycoplasmas. The presence of homologous multigene families in such phylogenetically distinct species, but not in the genomes of more closely related species, suggests that the families may have been transferred horizontally.

A hemagglutinin with an Mr of 67,000 (pMGA) from Mycoplasma gaUlisepticum S6 was purified by using monoclonal antibody affinity chromatography. Purified pMGA was treated with a number of enzymes, the resultant peptides were purified, and their amino acid sequence was determined by using an Applied Biosystems (model 471A) protein sequencer. The DNA sequence encoding two peptides was used to dictate the sequences of synthetic oligonucleotides which were used to screen a library ofEcoRI-cut M. gallisepticum DNA in pUC18. A clone reactive to both probes was isolated and found to contain a recombinant insert of 10 kb. The clone was mapped by using restriction endonucleases and fragments subcloned into pUC18 for DNA sequencing. Analysis of part of the DNA sequence revealed an open reading frame containing 1,941 nucleotides which encoded 647 amino acids. The amino terminus was preceded by a putative leader sequence of 25 amino acids. A promoter region preceding the putative start codon GUG was also located. This gene would encode a mature protein of 67,660 Da. There were a number of differences between the predicted amino acid sequence * Corresponding author.

Mycoplasma gallisepticum cell membranes were used to immunize mice to produce monoclonal antibodies to cell surface proteins. Three monoclonal antibodies were chosen for further characterization. All three reacted in immunoblots with an M. gaUlisepticum protein band of Mr approximately 67,000 (designated pMGA). By using immunoelectron microscopy, pMGA was shown to be located on the cell surface. When M. gallisepticum whole cells were treated with up to 250 pg of trypsin per ml for 30 min, the only major protein lost from the cell surface as judged by sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by Western immunoblot transfer was pMGA. Two of the pMGA-specific monoclonal antibodies inhibited hemagglutination of chicken erythrocytes by M. gallisepticum S6, suggesting a role for pMGA in the attachment of M. galisepticum to chicken erythrocytes. Sequencing the amino terminus of pMGA yielded 17 amino acids with no significant homology with the Mycoplasma pneumoniae attachment protein P1 or any other protein in the GenBank, Swiss-Prot, and EMBL data bases.

The E3 strain of E. coli was isolated in an outbreak of respiratory disease in broiler chickens, and experimental aerosol exposure of chickens to this strain induced disease similar to that seen in the field. In order to establish whether the virulent phenotype of this strain was associated with carriage of particular plasmids, four plasmid-cured derivatives, each lacking two or more of the plasmids carried by the wild-type strain, were assessed for virulence. Virulence was found to be associated with one large plasmid, pVM01. Plasmid pVM01 was marked by introduction of the transposon TnphoA, carrying kanamycin resistance, and was then cloned by transformation of E. coli strain DH5␣. The cloned plasmid was then reintroduced by conjugation into an avirulent plasmid-cured derivative of strain E3 which lacked pVM01. The conjugant was shown to be as virulent as the wild-type strain E3, establishing that this plasmid is required for virulence following aerosol exposure. This virulence plasmid conferred expression of a hydroxamate siderophore, but not colicins, on both strain E3 and strain DH5␣. Carriage of this plasmid was required for strain E3 to colonize the respiratory tracts of chickens but was not necessary for colonization of the gastrointestinal tract. However, the virulence plasmid did not confer virulence, or the capacity to colonize the respiratory tract, on strain DH5␣. Thus, these studies have established that infection of chickens with E. coli strain E3 by the respiratory route is dependent on carriage of a conjugative virulence plasmid, which confers the capacity to colonize specifically the respiratory tract and which also carries genes for expression of a hydroxymate siderophore. These findings will facilitate identification of the specific genes required for virulence in these pathogens.Lower-respiratory-tract infections are the most common disease syndrome associated with Escherichia coli in poultry. Although in extreme cases mortality can be over 20%, it is the high morbidity and associated loss in productivity which is responsible for the greatest economic loss (31).There is evidence to suggest that virulent strains of avian E. coli belong to a limited number of clone complexes (60,61) and that particular clones may be specific to particular manifestations of E. coli infection (47). A number of characteristics have been associated with virulence in avian E. coli, including colicin V production (22, 23, 50, 59), adhesins (17, 18, 20, 32, 46, 67), serum resistance (21,37,47,48,59,66), and iron sequestering (37,41,43,47,59,63), but specific attempts to establish the requirements of these factors for virulence are limited. While initial studies of avian E. coli led to the conclusion that certain serogroups, O1, O2, and O78 in particular, were more commonly associated with colibacillosis (27,28,29,33,35,36), the most prevalent serotypes vary with geographic location and many isolates are untypeable (3,6,12,19,38,45).Although plasmid-encoded virulence genes have been well investigated and described fo...