Polymorphism, genetic exchange and intragenic recombination of the aureolysin gene among Staphylococcus aureus strains

Sequence analyses support the conclusion that the aur gene occurs in two distinct types of related sequences. The aur gene was much more polymorphic but, at the same time, showed higher purifying selection than genes utilized for multilocus sequence typing (MLST). Gene trees constructed from aur and concatenated MLST genes revealed several putative assortative recombination events (i.e. entire aur gene exchanges) between divergent lineages of S. aureus. Evidence for intragenic recombination events (i.e. exchanges of internal aur segments) across aur genes was also found. The biochemical properties and substrate specificity of the two types of aureolysin purified to homogeneity were studied, revealing minor differences in their affinity to low molecular weight synthetic substrates.Although numerous nucleotide differences were identified between the aur alleles studied, our findings showed that a strong purifying selection is acting on the aur gene. Moreover, our study distinguishes between homologous exchanges of the entire aur gene (assortative recombination) between divergent S. aureus lineages and recombination events within aur genes.Proteases are important virulence factors for a variety of microbial pathogens and may contribute to tissue degradation and resistance to the host defense system. Staphylococcus aureus secretes a typical metalloprotease that is commonly referred to as aureolysin. This enzyme, which is a member of the thermolysin family of zinc-dependent metalloproteases, comprises a single chain of 301 amino acids that consists of a β-pleated N-terminal domain and an α-helical C-terminal domain [1]. In vitro, aureolysin activates prothrombin showing pseudocoagulase activity [2] and inactivates mammalian plasma protease inhibitors by cleavage [3,4]. Moreover, the enzyme has been suggested to contribute to the resistance of S. aureus to the innate immune system by degradation of the human antimicrobial peptide LL-37 [5] and inhibitory activity against