Expression of the pMGA genes of Mycoplasma gallisepticum is controlled by variation in the GAA trinucleotide repeat lengths within the 5 ' noncoding regions

We analyzed the segment of DNA which contains the expressed pMGA gene from one strain of Mycoplasma gallisepticum in normal (strain S6) cells and in c ells in which pMGA1.1 gene expression had ceased as a consequence of in vit ro culture in the presence of pMGA1.1-specific antibodies. Sequence analysi s of isolates lacking pMGA1.1 expression revealed that this gene, which is typically expressed, exhibited sequence changes within a region 5' to its p romoter. Specifically, pMGA1.1(+) cells contained a (GAA)(12) motif upstrea m of the promoter, whereas in pMGA1.1(-) cells the corresponding region con tained a (GAA)(10) motif; when such cells were grown in medium no longer co ntaining pMGA-specific antibodies, pMGA1.1 was reexpressed and the 5' (GAA) (12) motif was restored. Two other genes, pMGA1.9 and pMGA1.2, were also sh own to acquire a (GAA)(12) motif in clones which expressed these genes. The results imply the evolution by the pMGA genes of M. gallisepticum of a nov el transcriptional requirement which facilitates rapid and reversible switc hes in the pMGA expression pattern.