IS911 partial transposition products and their processing by the Escherichia coli RecG helicase

Insertion of bacterial insertion sequence IS911 can often be directed to sequences resembling its ends. We have investigated this type of transposition and shown that it can occur via cleavage of a single end and its targeted transfer next to another end. The single end transfer (SET) events generate branched DNA molecules that contain a nicked Holliday junction and can be considered as partial transposition products. Our results indicate that these can be processed by the Escherichia coli host independently of IS911-encoded proteins. Such resolution depends on the presence of homologous DNA regions neighbouring the cross-over point in the SET molecule. Processing is often accompanied by sequence conversion between donor and target sequences, suggesting that branch migration is involved. We show that resolution is greatly reduced in a recG host. Thus, the branched DNA-specific helicase, RecG, involved in processing of potentially lethal DNA structures such as stalled replication forks, also intervenes in the resolution of partial IS911 transposition products.