Regulated site‐specific recombination of the she pathogenicity island of Shigella flexneri

Abstract

SummaryThe she pathogenicity island (PAI) is a chromosomal, laterally acquired, integrative element of Shigella flexneri that carries genes with established or putative roles in virulence. We demonstrate that spontaneous, precise excision of the element from its integration site in the 3 ¢ ¢ ¢ ¢ terminus of the pheV tRNA gene is mediated by an integrase gene ( int ) and a gene designated rox (regulator of excision), both of which are carried on the she PAI. Integrase-mediated excision occurs via recombination between a 22 bp sequence at the 3 ¢ ¢ ¢ ¢ terminus of pheV and an imperfect direct repeat at the pheV -distal boundary of the PAI. Excision leads to the formation of a circular episomal form of the PAI, reminiscent of circular excision intermediates of other mobile elements that are substrates for lateral transfer processes such as conjugation, packaging into phage particles and recombinase-mediated integration into the chromosome. The circle junction consists of the pheV -proximal and pheV -distal boundaries of the PAI converging on a sequence identical to 22 bp at the 3 ¢ ¢ ¢ ¢ terminus of pheV . The isolated circle was transferred to Escherichia coli where it integrated specifically into phe tRNA genes, as it does in S. flexneri , independently of recA . We also demonstrate that Rox stimulates, but is not essential for, excision of the she PAI in an integrase-dependent manner. However, Rox does not stimulate excision by activating the transcription of the she PAI integrase gene, suggesting that it has an excisionase function similar to that of a related protein from the P4 satellite element of phage P2.

Comparative Genome Analysis of “CandidatusPhytoplasma australiense” (Subgrouptuf-Australia I;rp-A) and “Ca. Phytoplasma asteris” Strains OY-M and AY-WB

“…Phytoplasma australiense" PMUs may have been laterally transferred and accumulated throughout the genome is unknown, since the features that characterize composite transposons, such as inverted repeat regions, were not found. If the PMUs are mobile, they may rely on helper elements to provide conjugative transfer, or they may package the mobile units into phage particles (45,71,81).…”

Excision of the Shigella Resistance Locus Pathogenicity Island in Shigella flexneri Is Stimulated by a Member of a New Subgroup of Recombination Directionality Factors

“…Like many PAIs, the SRL PAI encodes an integrase that is homologous to members of the P4 family of integrases (13). Integrases mediate PAI integration into the chromosome (19,26) and are also essential for PAI excision (11,20,27). However, little is known about how the excision of PAIs is regulated.…”

“…2). Of the new subgroup, Rox she , YPO1904, and Vis have been shown experimentally to have a role in excision (10,20; D. Ghisotti, personal communication).…”

Horizontal Transfer of the High-Pathogenicity Island ofYersinia pseudotuberculosis

“…The 500-kb symbiosis island of Mesorhizobium loti is transferable at a frequency of 10 Ϫ7 to three Mesorhizobium species (42). The transmissibility of this island has been associated with the presence of the trb operon and the traG gene, which display homology to genes required for the conjugative transfer of some plasmids (43).The hypothesis that genomic islands could be transmitted by phages has been put forward because, as with bacteriophages, these islands are most often inserted into tRNA genes, they excise from the host chromosome by site-specific recombination between flanking direct repeats homologous to phage attachment sites, and their excision is mediated by a phagelike integrase and sometimes a recombination directionality factor (22,25,36). Pathogenicity islands (PAIs) are a subset of genomic islands characterized by their capacity to encode virulence functions (17).…”

“…The hypothesis that genomic islands could be transmitted by phages has been put forward because, as with bacteriophages, these islands are most often inserted into tRNA genes, they excise from the host chromosome by site-specific recombination between flanking direct repeats homologous to phage attachment sites, and their excision is mediated by a phagelike integrase and sometimes a recombination directionality factor (22,25,36). Pathogenicity islands (PAIs) are a subset of genomic islands characterized by their capacity to encode virulence functions (17).…”