Inhibition of F plasmid replication in htpR mutants of Escherichia coli deficient in sigma 32 protein.

The Escherichia coli htpR (= hin, rpoH) mutants are defective in the induction of heat-shock proteins due to a deficiency in sigma 32 and are unable to grow at high temperature. We found that these mutants are also defective in supporting replication of certain plasmids including F and mini-F. When a htpR mutation is introduced into an F' strain, the F' plasmid is effectively excluded. Similarly, when an F' or mini-F plasmid is introduced into htpR mutant cells, transconjugant or transformant clones are obtained at low frequencies and the plasmid is rapidly lost upon subsequent growth in a non-selective medium. In htpR amber mutants carrying a temperature-sensitive suppressor, mini-F replication occurs normally at 30 degrees C, but is inhibited upon transfer to 40 degrees C where the suppressor tRNA is inactivated. A temperature-resistant "pseudo-revertant" of the htpR6 (amber) mutant, that exhibits apparently normal induction of the major heat-shock proteins in the absence of functional sigma 32, fails to support mini-F replication at 40 degrees C, suggesting that inhibition of mini-F replication is not a secondary consequence of the defective induction of the major heat-shock proteins. It is proposed that the function of the sigma 32 protein is directly required for F plasmid replication.