No abstract

No abstract

The rep gene function of Escherichia coli is essential for the replication of P2 and 4X174 double-stranded deoxyribonucleic acid (DNA). Compared with isogenic rep+ strains, rep mutants show the following characteristics: larger cell size, more DNA per cell, and a slightly lower DNA/mass ratio. The replicating rep chromosomes show a steeper gradient of marker frequencies and contain more replicating forks per chromosome. The nucleoid body of rep mutants sediments faster and contains more DNA. We deduce that the rep function is required for the "normal" replication of the E. coli chromosome and that in its absence the E. coli chromosome replicates in an altered manner, perhaps involving slower-moving replicating forks. Denhardt et al. (13) originally isolated rep mutants of Escherichia coli as strains unable to support growth of bacteriophage 4X174. The

No abstract

~ ~~ ~~Gel electrophoresis of DNA from 95 clinical isolates of Shigella sonnei and Shigella JIexneri resistant to antibiotics revealed a heterogeneous plasmid population. Most of the plasmids were smaller than 6 megadaltons (Mdal). Six S. sonnei isolates with the most common antibiotic resistance pattern were characterized. They had two plasmids in common : one was a self-transmissible Fif plasmid of 46 Mdal encoding tetracycline resistance, while the other was a 5.5 Mdal non-conjugative plasmid encoding resistance to streptomycin and sulphafurazole. In addition, several cryptic plasmids ranging in size from 1.0 to 24.5 Mdal were present. Mobilization of the 5.5 Mdal SuSm plasmid and a 1.0 Mdal cryptic plasmid was demonstrated with all six S. sonnei isolates during conjugation. This mobilization was mediated by the 46 Mdal self-transmissible Fif R plasmid and also by a 24-5 Mdal Fiplasmid carrying no known drug resistance determinants.