Resolvase-like serine recombinase mediates integration/excision in the bacteriophage φRSM

Abstract

ϕRSM1, a filamentous phage infecting Ralstonia solanacearum, encodes an open reading frame (ORF14) that exhibits significant homology to members of the resolvase/invertase subfamily of site-specific serine recombinases. Similar prophages are found in the genomes of various strains of R. solanacearum, R. pickettii, and Burkholderia pseudomallei. We have characterized the integrative and excisive recombination reactions mediated by the ϕRSM1 integrase using in vivo assays. An E. coli plasmid containing the RSM1 orf14 and attP sequences (pT-orf14-attP) was shown to integrate into the attB sequence of R. solanacearum cells. Intermolecular recombination between pT-orf14-attP and pS-attB (a R. solanacearum plasmid containing attB) also occurred in R. solanacearum cells. The products of attP/attB recombination, i.e., attL and attR, were exactly identical to the sequences found in the prophage ϕRSM in R. solanacearum strains. Intramolecular recombination between the attL and attR sequences (excisive recombination) on the pRecomb plasmid was also shown to occur in an orf14-dependent manner. This is the first evidence that a small serine recombinase from the resolvase/invertase group may function in integrative and excisive recombination for filamentous phages.