Staphylococcal plasmid cointegrates are formed by host- and phage-mediated general rec systems that act on short regions of homology.

Cointegrates involving pairs of compatible staphylococcal plasmids can be isolated either by co-selection during transduction (Novick et al. 1981) or by selection for survival at the restrictive temperature of a thermosensitive, replication defective plasmid in the presence of a stable one. Cointegrates are formed by recombination at two specific sites, RSA and RSB. RSB is present on each of six plasmids analyzed, namely pT181, pE194, pC194, pS194, pUB110, and pSN2, and RSA is present on two of these, pT181 and pE194. In this communication, it is shown that the RS represent short regions of homology (RSA is some 70 bp in length and RSB is about 30) embedded in largely non-homologous contexts and that the crossovers take place within these homologous regions. The pT181 and pE194 RSA sequences contain several mismatches which permit the localization of the crossover events to several different sites within the overall RS segment. The recombination system involved is therefore general (homology-specific) rather than site-specific (sequence-specific). Mismatches included within the crossover region are always corrected to the pT181 configuration. The cointegrates are therefore formed by a relatively efficient general rec system that recognizes short regions of homology and gives rise to Holliday junctions that probably involve very short heteroduplex overlaps. The sequence results are consistent with asymmetric single-strand invasion of a contralateral gap with nucleotide conversion by copying. It is noted that RSB has substantial homology with the par sequence of plasmid pSC101, suggesting that it may be involved in plasmid partitioning.