For absent friends: life without recombination in mutualistic gamma-proteobacteria

Almost all cellular organisms employ RecA orthologues to guide the strand invasion reactions necessary for DNA recombination and repair. One of the few exceptions to this orthodoxy is a group of gamma-proteobacteria flourishing in obligate intracellular symbiosis with insects and deep-sea clams. The apparent inability of these bacteria to commence the recombinational exchange process seems to confer genetic stability by preventing any further rearrangements or lateral transfer events. Although debate has centred on the absence of selected recombination functions and their impact on a fixed genomic architecture, no explanation has been offered for how bacteria survive the loss of such an integral DNA repair system. This question is addressed here by speculating on how the current repertoire of recombinases in symbiotic bacteria could enable recovery from potentially lethal injuries to the DNA template. Depending on which functions remain, several different options are plausible. The possibility that specific defects in recombination encourage radical genome erosion in mutualistic endosymbionts and other intracellular bacteria is discussed.