SMALL CRYPTIC PLASMIDS OF MULTIPLASMID, CLINICAL ESCHERICHIA-COLI

Clinical isolates of Escherichia coil were found to host a multiplicit y of plasmids. These were resolved from plasmid gel profiles, from the properties of various transconjugants and transformants of E. coli DH 1, by the topoisomerase I relaxation of covalently closed circle plasm id DNA, by electron microscopy, and by the determination of their comp atibilities. The majority of these were unusually small, cryptic plasm ids (SCPs). From one strain, KL4, 13 electrophoretic bands were resolv ed to five plasmids, three of which were SCPs. SCPs were phenotypicall y barren, and the smallest of these, pKL1, contained barely enough inf ormation for self-replication, A derivative of pKL1, pKL1Km, in which the transposon was restricted to a small 350-bp region, was stably mai ntained in Shigella, Salmonella, Serratia, and Citrobacter species and its replication was polA independent. pKL1 encoded only a single prot ein, RepA (M(r) 17960), which specifically bound to pKL1 DNA. No appar ent homologies with other RepA protein sequences could be detected. Th us the SCP, pKL1, is a novel minimal plasmid replicon encoding only en ough information to ensure perpetuation. A hypothesis is presented des cribing SCPs as a class of selfish DNA that persists simply due to its ability to replicate and to its stability based on high copy number. (C) 1997 Academic Press.