The Escherichia coli retrons Ec67 and Ec86 replace DNA between the cos site and transcription terminator of a 186-related prophage

Retrons are unusual, reverse transcriptase-encoding elements found in bacteria. Although there are a number of indications that retrons are mobile elements, their transposition has not been observed. The Escherichia coli retrons Ec67 and Ec86 are different retrons inserted at the same site and we have further characterized this site in search of clues to the mechanism of retron transposition. We confirm, by extending previous sequence analysis, that Ec67 and Ec86 are inserted into prophages related to coliphage 186. Comparison with the recently published sequence of the 186 96-2% region indicates that the retrons have replaced similar to 180 bp of DNA between the phage cohesive end site (cos) and the transcription terminator of a phage DNA-packaging gene. These features - DNA replacement at the insertion site and the location of retron junctions near transcription terminators or DNA cleavage sites - are shared with other retrons and suggest ways in which retron transposition might have occurred. (C) 1996 Academic Press, Inc.