Analysis of a retron EC86 and EC67 insertion site in Escherichia coli

The Escherichia coli clinical strain 49 contains the integration sites for the retrons EC86 and EC67 but not the retrons themselves. To compare the chromosomal structures before and after retron integration, the DNA sequence of the integration site in strain 49 was determined and compared to the corresponding sequences in strains containing EC86 and EC67. The results suggested that when these retrons inserted into the E. coli chromosome they replaced a 3.5-kb fragment of chromosomal DNA. It is proposed that the replacement of preexisting DNA by a retron may be a general mechanism for retron integration, since in the three examples in which the integration sites are known, the retrons appear to have integrated into the chromosome by replacing a preexisting DNA segment.