The plasmid pE194 (3.7 kilobases) is capable of integrating into the genome of the bacterial host Bacillus subtilis in the absence of the major homology-dependent RecE recombination system. Multiple recombination sites have been identified on both the B. subtilis chromosome and pE194 (J. Hofemeister, M. Israeli-Reches, and D. Dubnau, Mol. Gen. Genet. 189:58-68, 1983). The B. subtilis chromosomal recombination sites were recovered by genetic cloning, and these sites were studied by nucleotide sequence analysis. Recombination had occurred between regions of short nucleotide homology (6 to 14 base pairs) as indicated by comparison of the plasmid and the host chromosome recombination sites with the crossover sites of the integration products. Recombination between the homologous sequences of the plasmid and the B. subtilis genome produced an integrated pE194 molecule which was bounded by direct repeats of the short homology. These results suggest a recombination model involving a conservative, reciprocal strand exchange between the two recombination sites. A preferred plasmid recombination site was found to occur within a 70-base-pair region which contains a GC-rich dyad symmetry element. Five of seven pE194-integrated strains analyzed had been produced by recombination at different locations within this 70-base-pair interval, located between positions 860 and 930 in pE194. On the basis of these data, mechanisms are discussed to explain the recombinational integration of pE194.Several intramolecular and intermolecular genetic recombination events which do not require extensive nucleotide sequence homology have been described for Bacillus subtilis (18,23,25,34). These rare events are independent of the major homologous DNA recombination system, the RecE pathway, of B. subtilis and produce stable rearrangements in the cellular genetic material. Often these genetic rearrangements are accompanied by changes in the phenotype of the host cell and provide a means of evolving new traits, such as the stable acquisition of antibiotic resistance determinants. The integration of plasmid pE194 into the B. subtilis genome (19) represents an example of recE-independent intermolecular recombination. Integration of pE194 places the ermC antibiotic resistance determinant into the B. subtilis chromosome. This gene is subsequently inherited as a chromosomal marker.pE194 is a 3.7-kilobase (kb) multicopy plasmid, originally isolated from Staphylococcus alureus (22), that was stably introduced into B. slubtilis by genetic transformation (13). The entire nucleotide sequence is known for pE194 (20). The ermC gene encodes an inducible 29-kilodalton product which is responsible for mediating resistance to the macrolidelincosamide-streptogramin B (MLS) group of antibiotics (45). pE194 is known to be naturally temperature sensitive for replication (39), being unable to replicate autonomously at growth temperatures exceeding 43°C. The plasmid-encoded trans-acting RepF protein is required for initiation of pE194 replication (16,44), which is tho...