The RuvA homologues from Mycoplasma genitalium and Mycoplasma pneumoniae exhibit unique functional characteristics

The DNA recombination and repair machineries of Mycoplasma genitalium and Mycoplasma pneumoniae differ considerably from those of gram-positive and gram-negative bacteria. Most notably, M. pneumoniae is unable to express a functional RecU Holliday junction (HJ) resolvase. In addition, the RuvB homologues from both M. pneumoniae and M. genitalium only exhibit DNA helicase activity but not HJ branch migration activity in vitro. To identify a putative role of the RuvA homologues of these mycoplasmas in DNA recombination, both proteins (RuvAMpn and RuvAMge, respectively) were studied for their ability to bind DNA and to interact with RuvB and RecU. In spite of a high level of sequence conservation between RuvAMpn and RuvAMge (68.8% identity), substantial differences were found between these proteins in their activities. First, RuvAMge was found to preferentially bind to HJs, whereas RuvAMpn displayed similar affinities for both HJs and single-stranded DNA. Second, while RuvAMpn is able to form two distinct complexes with HJs, RuvAMge only produced a single HJ complex. Third, RuvAMge stimulated the DNA helicase and ATPase activities of RuvBMge, whereas RuvAMpn did not augment RuvB activity. Finally, while both RuvAMge and RecUMge efficiently bind to HJs, they did not compete with each other for HJ binding, but formed stable complexes with HJs over a wide protein concentration range. This interaction, however, resulted in inhibition of the HJ resolution activity of RecUMge.

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