CONDUCTION OF PEC22, A PLASMID CODING FOR MR-CENTER-DOT-ECOT22I, MEDIATED BY A RESIDENT TN3-LIKE TRANSPOSON, TN5396

pEC22 is a small plasmid that encodes the restriction-modification sys tem MR.EcoT22I. Restriction and functional analysis of the plasmid ide ntified the positions of genes encoding that system. The plasmid is ab le to be conducted by conjugal plasmids, a process mediated by a trans poson contained within pEC22. This cryptic transposon, called Tn5396, was isolated from pEC22 and partially sequenced. The sequence of Tn539 6 is for the most part typical of transposons of the Tn3 family and is most similar to that of Tn1000. The transposon differs from closely r elated transposons in that it lacks well-conserved sequences in the in verted-repeat region and has an unusually long terminal inverted repea t. Consideration of regions of internal sequence similarity in this an d other transposons in the Tn3 family supports a theory of the mechani sm by which the ends of Tn3-like transposons may maintain substantial identity between their inverted repeats over the course of evolutionar y time.