Occurrence Plasticity and Evolution of the vpma Gene Family a Genetic System Devoted to High-Frequency Surface Variation in Mycoplasma agalactiae

摘要

Mycoplasma agalactiae, an important pathogen of small ruminants, exhibits a very versatile surface architecture by switching multiple, related lipoproteins (Vpmas) on and off. In the type strain, PG2, Vpma phase variation is generated by a cluster of six vpma genes that undergo 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 between two loci located 250 kbp apart. Loci I and II include 16 and 7 vpma genes, respectively, with all vpma genes of locus II being duplicated at locus I. Several Vpmas displayed a chimeric structure suggestive of homologous recombination, and a global proteomic analysis further indicated that at least 13 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 for strain 5632 than for 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 elements 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.