Functional characterization of the RuvB homologs from Mycoplasma pneumoniae and Mycoplasma genitalium

Homologous recombination between repeated DNA elements in the genomes of Mycoplasma species has been hypothesized to be a crucial causal factor in sequence variation of antigenic proteins at the bacterial surface. To investigate this notion, studies were initiated to identify and characterize the proteins that form part of the homologous DNA recombination machinery in Mycoplasma pneumoniae as well as Mycoplasma genitalium. Among the most likely participants of this machinery are homologs of the Holliday junction migration motor protein RuvB. In both M. pneumoniae and M. genitalium, genes have been identified that have the capacity to encode RuvB homologs (MPN536 and MG359, respectively). Here, the characteristics of the MPN536- and MG359-encoded proteins (the RuvB proteins from M. pneumoniae strain FH [RuvBFH] and M. genitalium [RuvBMge], respectively) are described. Both RuvBFHand RuvBMgewere found to have ATPase activity and to bind DNA. In addition, both proteins displayed divalent cation- and ATP-dependent DNA helicase activity on partially double-stranded DNA substrates. The helicase activity of RuvBMge, however, was significantly lower than that of RuvBFH. Interestingly, we found RuvBFHto be expressed exclusively by subtype 2 strains of M. pneumoniae. In strains belonging to the other major subtype (subtype 1), a version of the protein is expressed (the RuvB protein from M. pneumoniae strain M129 [RuvBM129]) that differs from RuvBFHin a single amino acid residue (at position 140). In contrast to RuvBFH, RuvBM129displayed only marginal levels of DNAunwinding activity. These results demonstrate that M. pneumoniae strains (as well as closely related Mycoplasma spp.) can differ significantly in the function of components of their DNA recombination and repair machinery.

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