Recombination Phenotypes of Escherichia coli greA Mutants

Escherichia coli mutants bearing substitutions of the active site acidic residues of the transcription elongation factor GreA, D41N and E44K, were isolated as suppressors of growth inhibition by a toxic variant of the bacteriophage lambda Red-beta recombination protein. These mutants, as well as a D41A greA mutant and a greA deletion, were tested for proficiency in recombination events. The mutations were found to increase the efficiency of RecA-RecBCD-mediated and RecA-Red-mediated recombination, which are replication-independent, and to decrease the efficiency of replication-dependent Red-mediated recombination.These observations provide new evidence for a role of GreA in resolving conflicts between replication and transcription.The GreA protein is an evolutionarily-conserved bacterial transcriptional factor, which interacts directly with RNA polymerase. Binding of GreA to paused and backtracked transcriptional complexes induces RNA polymerase to hydrolyze the last few nucleotide residues at the 3' end of the nascent transcript protruding beyond the enzyme's catalytic center, and resume transcription, in vitro [1] and in vivo [2,3]. The extensively characterized interaction between RNA polymerase and GreA leads most simply to the idea that GreA functions in transcriptional regulation. Consistent with this idea, E. coli lacking GreA protein exhibits altered levels of expression of some of its genes [4,5], as well as of the transcript encoding bacteriophage λ proteins which are synthesized late in infection [6]. However, the magnitude of the gene expression effects is small, and a clear understanding of GreA's biological function remains to be obtained. Some recent studies have provided evidence that GreA may have a role in resolving conflicts between replication and transcription [7,8].In this study, greA mutants are shown to have effects on homologous recombination which can be most readily explained by a replication fork-preserving function of wild type GreA. The