Gene Families Encoding Phase- and Size-Variable Surface Lipoproteins of Mycoplasma hyorhinis

A prototype family of seven genes encoding the variable surface lipoproteins (Vlps) of Mycoplasma hyorhinis is characterized in the pathogenic SK76 strain, using long-range PCR to amplify and analyze the single chromosomal region containing expressed genes vlpA to -G, each of which is subject to phase and size variation. Smaller families of vlp genes in subclones of SK76 or in another strain of M. hyorhinis, GDL, can be attributed to deletions of specific vlp genes from the prototype array described here. Two genes, vlpA and the newly revealed vlpG, contain repeat motifs in their 3′ coding regions that differ from the short tandem repeats in other vlp genes yet retain structural features common to all vlp gene products. SK76 and GDL vlp gene families are similarly organized and show sequence similarity between corresponding individual vlp genes. In light of the extensive potential for diversity within the vlp gene system, such conservation provides a provisional basis to hypothesize that vlp genes may exist in specific arrays that endow selected functions while retaining common structural features required during phase-variable expression of this set of gene products.