a1 Department of Physiology, Harvard Medical School, 25 Shattuck Street, Boston, Mass. 02115, U.S.A.
a2 Institute of Animal Genetics, West Mains Road, Edinburgh EH9 3JN, Scotland
Klebsiella aerogenes V9A carrying a lac plasmid in addition to its chromosomal operon showed strongly positive fermentation of lactose on MacConkey lactose agar plates, and was found to transport the lactose analogue thiomethyl-β-galactoside (TMG) at a rapid rate. The strain that had been freed of the plasmid showed moderate transport due to the chromosomal lac operon. When a plasmid bearing a mutation in lac Y was inserted into a strain with a normal Y gene, the resulting diploid became lactose-negative in phenotype. The presence of E. coli F′lac factors that carried lac Y mutations, whether deletions or missense or nonsense mutations, also rendered lac Y+ Klebsiella lactose-negative. Such diploids, after growth in 1% lactose, transported TMG at a much lower rate than the corresponding plasmid-free lac Y+ Klebsiella. However, this interference by lac Y− plasmids with the expression of the chromosomal lac Y gene was not seen when cells were induced with IPTG or when the chromosomal and plasmid lac operons were both constitutive. It was found that this effect of the plasmids was dependent on their possessing an intact lacZ gene.
(Received November 08 1977)