Hybrids composed of phage M13 and plasmid pHV33 were used to study the formation of deletions in Escherchia coli. Eighty to ninety percent of the deletion endpoints were at the position of the nick introduced into the M13 replication origin by the phage gene II protein. This suggests the existence of a novel mechanism of illegitimate recombination.Recombination between sequences with little or no homology was termed "illegitimate" by Franklin (1) ref. 14), and AB1157 (thrAl leu-6 thi-) proA2 his4 argE3 lacYl galK2 ara-14 xyl-S mtl-i tsx-33 rpl-31 supE44; J.Clark, University of California, Berkeley). The Rec-strains were all isogenic to AB1157. They were rendered hsdR by conjugation with GY2098 (HfrH) after cotransfer of hsdR and thr'. recA strains JC5495 and JC5547 were rendered Rec' prior to conjugation by introduction of a RecAI X phage. This phage was eliminated from the exconjugants by segregation. All the Rec-strains harbored F'lacIqZ M15 pro' plasmid (15). The Rec-phenotype was routinely tested by sensitivity to ultraviolet light. The Rep-strain D162 was rendered hsdR by conjugation with a leu+ revertant of GY2098 after cotransfer of hsdR and leu+ markers. It harbored pOXKm plasmid (M. Chandler, Toulouse University).The plasmid pHV33 is composed of pBR322 and pC194 (16-18). The plasmids pHV672 (19) and pHV673 were constructed by joining EcoRI-cleaved M13 mp2 phage (20) to pHV33; they were isolated in B. subtilis. pHV698 was constructed by replacing the small pBR322 Cla I-EcoRI fragment with the 540-base-pair (bp) M13 mp2 Cla I-EcoRI fragment that carries the phage replication origin. pHV702 was obtained by inserting pC194 into the HindIII site of pHV698. R199 and R229 are two fl derivatives (21,22) (28,29) and was purified by chromatography on hydroxyapatite columns (30). For analytical purposes, plasmid DNA was extracted from 1.5-ml overnight cultures (31). The terminal-labeling method was used for DNA sequencing (32). Transformation of B. subtilis and E. coli competent cells and plasmid transduction have been described (28,33,34).
RESULTSIsolation of Deletion Plasmids. Plasmid pHV33, composed ofthe E. coli plasmid pBR322 and the Staphylococcus aureus plasmid pC194, is viable in E. coli (18). We have joined the Abbreviations: bp, base pair(s); kb, kilobase(s).
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