Abstract
Mycoplasma agalactiae, an important pathogen of small ruminants, exhibits antigenic diversity by switching the expression of multiple surface lipoproteins called Vpmas (Variable proteins of M. agalactiae). Although phase variation has been shown to play important roles in many host–pathogen interactions, the biological significance and the mechanism of Vpma oscillations remain largely unclear. Here, we demonstrate that all six Vpma proteins are expressed in the type strain PG2 and all undergo phase variation at an unusually high frequency. Furthermore, targeted gene disruption of the xer1 gene encoding a putative site-specific recombinase adjacent to the vpma locus was accomplished via homologous recombination using a replicon-based vector. Inactivation of xer1 abolished further Vpma switching and the ‘phase-locked’ mutants (PLMs) continued to steadily express only a single Vpma product. Complementation of the wild-type xer1 gene in PLMs restored Vpma phase variation thereby proving that Xer1 is essential for vpma inversions. The study is not only instrumental in enhancing our ability to understand the role of Vpmas in M. agalactiae infections but also provides useful molecular approaches to study potential disease factors in other ‘difficult-to-manipulate’ mycoplasmas.
Highlights
Reversible high-frequency changes of bacterial cell surfaces via phase variation are thought to increase resistance against host defences or enhance survival under stressful environmental conditions (Henderson et al, 1999; van der Woude and Baumler, 2004)
We demonstrate that all six Vpma proteins are expressed in the type strain PG2 and all undergo phase variation at an unusually high frequency
Through the use of specific polyclonal antibodies raised against all six Vpma proteins we show that all six Vpmas are expressed within a population of M. agalactiae type strain PG2, that this expression is on the surface of the cells, and that all Vpmas exhibit an unexpectedly high frequency of phase variation which was underestimated in previous studies (Glew et al, 2000)
Summary
Reversible high-frequency changes of bacterial cell surfaces via phase variation are thought to increase resistance against host defences or enhance survival under stressful environmental conditions (Henderson et al, 1999; van der Woude and Baumler, 2004). The 5′ untranslated regions and those encoding the signal peptide are conserved within the vpma gene family and share a high identity to the equivalent regions of vsp genes forming a similar multigene phase-variable system in the bovine pathogen Mycoplasma bovis (Lysnyansky et al, 1999). Both vsp and vpma genes contain repeated sequences, exhibit the same lipoprotein cleavage motif (AAKC) and encode similar short cytadherence epitopes (Glew et al, 2002). The knowledge gained thereof might provide important clues to understand M. bovis infections which cause major economic losses worldwide (Nicholas and Ayling, 2003)
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