Studies on a naturally occurring R factor harbouring a chromosomal nutritional marker

A naturally occurring R factor which carried resistance determinants to tetracycline and chloramphenicol as well as a marker for lactose fermentation was found in a strain of Klebsiella pneumoniae isolated from sewage. The TcCmlac+ factor was transferred by conjugation to E. coli K12. It showed no recombination or stable coexistence with R factors of the fi- class. Studies in recombination-deficient (rec-) strains of E. coli K12 showed that the TcCmlac+ factor is an autonomously replicating plasmid. Preparative ultracentrifugation and electron microscopic studies revealed the presence of more than one molecular species. Average contour lengths for two of these species were 11,20 +/- 1,55 mum and 16,14 +/- 1,45 mum, which correspond to molecular weights of about 23,18 X 10(6) and 34,63 X 10(6) daltons, respectively.