A novel mechanism for control of antigenic variation in the haemagglutiningene family of Mycoplasma synoviae

High-frequency phase and antigenic variation of homologous lipoprotein haem agglutinins has been seen in both the major avian mycoplasma pathogens, Myc oplasma synoviae and Mycoplasma gallisepticum. The expression and, hence, a ntigenic variation of the pMGA gene family (encoding these lipoproteins in M. gallisepticum) is controlled by variation in the length of a trinucleoti de repeat motif 5' to the promoter of each gene. However, such a mechanism was not detected in preliminary observations on M. synoviae. Thus, the basi s for control of variation in the vlhA gene family (which encodes the homol ogous haemagglutinin in M. synoviae) was investigated to enable comparison with its homologue in M. gallisepticum and with other lipoprotein gene fami lies 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. Comparis on 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 t he conserved 5' coding sequence occurred as a single copy, whereas the rema inder of the coding sequence occurred as multiple copies. A 9.7 kb fragment of the genome was found to contain eight tandemly repeated regions partial ly homologous to vlhA, all lacking the putative promoter region and the sin gle-copy 5' end of vlhA, but extending over one of four distinct overlappin g 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 the y differed in their reaction with monoclonal antibodies specific for this r egion. These data suggest that the control of vlhA antigenic variation in M . synoviae is achieved by multiple gene conversion events using a repertoir e of coding sequences to generate a chimeric expressed gene, with the great est potential for variation generated in the region encoding the haemagglut inin. Thus, completely distinct mechanisms have been adopted to control ant igenic variation in homologous gene families.