Occurrence, plasticity and evolution of the vpma gene family, a genetic system devoted to high frequency surface variation in Mycoplasma agalactiae

Abstract

Mycoplasma agalactiae, an important pathogen of small ruminants, exhibits a very versatile surface architecture by switching ON-OFF multiple, related lipoproteins (Vpmas). In the type strain, PG2, Vpma-phase variation is generated by a cluster of 6 vpma genes that undergoes frequent DNA rearrangements via site-specific recombination. To further comprehend the degree of diversity that can be generated at the M. agalactiae surface, the vpma gene repertoire of a field strain, 5632, was analyzed and shown to contain an extended repertoire of 23 vpma genes distributed onto two loci located 250 kbp apart. Loci I and II include 16 and 7 vpmas, respectively, with all vpmas of locus II being duplicated at locus I. Several Vpmas displayed a chimeric structure suggestive of homologous recombination and a global proteomic analyses further indicate that at least 13 out of the 16 Vpmas can be expressed by the 5632 strain. Because a single promoter is present in each vpma locus, concomitant Vpma expression can occur in a strain with duplicated loci. Consequently, the number of possible surface combinations is much higher in 5632 compared to the type strain. Finally, our data suggested that insertion sequences are likely to be involved in 5632 vpma locus duplication at a remote chromosomal position. The role of such mobile genetic element in chromosomal shuffling of genes encoding major surface components may have important evolutionary and epidemiological consequences for pathogens such as mycoplasmas that have a reduced genome and no cell wall.