Variation of vlhA gene in Mycoplasma synoviae clones isolated from chickens.

Mycoplasma synoviae synthesizes haemagglutinin VlhA, which cleaves into the N-terminal part, a lipoprotein MSPB, and a C-terminal part MSPA. Previous studies have shown that the 3'-end of the expressed vlhA gene can recombine with vlhA pseudogenes in a process called gene conversion, but there have been no data about diversification of the expressed vlhA gene in M. synoviae populations replicating in chickens. Following intratracheal inoculation with the M. synoviae strain ULB 02/T6, which showed only minor vlhA gene variation prior to inoculation, we investigated temporal changes in MSPB epitopes defined by monoclonal antibodies (mAbs) 3B4 and 50, as well as diversification of the vlhA gene sequence in M. synoviae populations recovered from chicken tracheas. In cultures isolated 8 and 18 days post inoculation (p.i.), most colonies showed variation of MSPB epitopes for mAbs 3B4 and 50. They also changed 3'-end vlhA gene sequences. Further diversity of the vlhA gene occurred in cultures isolated 8 weeks and 5 months p.i. The vlhA gene sequences from isolated cultures shared only 65 to 80% sequence identity with vlhA gene of the inoculated ULB 02/T6 culture. Notably, in most of those cultures their vlhA gene sequences contained stop codons potentially causing premature terminations of translation. Interestingly, in one culture isolated 8 weeks p.i. (clone T6-8W/IT2A) the 3'-vlhA gene sequence was identical in the last 1140 bases to that of the first vlhA pseudogene positioned the most far (upstream) of the expressed vlhA gene. This is the first demonstration of temporal diversity of the vlhA gene in M. synoviae populations isolated from chicken tracheas.