Cellular control of conjugation in Escherichia coli K12 : Effect of chromosomal cpx mutations on F-plasmid gene expression

Abstract

DNA donor activity and surface exclusion of Escherichia coli F + and Hfr strains require expression of both F-plasmid and chromosomal genes. The plasmid genes are contained in the 35,000 base tra region, where most of them are organized as a co-regulated gene block designated the traY → Z operon. The chromosomal genes have been identified among chromosomal mutants that fail to express donor activity and surface exclusion, even when they carry normal F-plasmid DNA. We show here that mutations in two chromosomal genes, cpxA and cpxB, together reduce the abundance of tra operon mRNA to 15% or less of the value in otherwise isogenic cpxA + cpxB + cells. The cpxB1 mutation alone had no effect on the tra operon messenger RNA level, in agreement with previous evidence that this allele by itself is cryptic. We attribute the effect of both cpx mutations on tra operon mRNA to a transcriptional defect resulting from the inability of mutant cells to accumulate the traJ gene product, a 24,000 M r outer membrane protein that is also required for efficient tra operon expression in vivo. Ultraviolet light-irradiated cpxA2 cpxB1 mutant cells infected with a λp( traJ) transducing bacteriophage that contains an intact traJ gene and its normal control sequences failed to accumulate the TraJ protein as a 24,000 M r polypeptide, whereas cpxA + cpxB1 cells, otherwise isogenic, did. In the same experimental system, both cpxA2 cpxB1 and cpxA + cpxB1 cells infected with the λp( traJ) bacteriophage accumulated comparable levels of RNA complementary to traJ. Moreover, cpxA2 cpxB1 and cpxA + cpxB1 cells synthesized comparable levels of β-galactosidase from a traJ-lacZ protein fusion. These results show that the cpx mutations do not reduce transcription or translation initiation at traJ sequences. They define a new cellular contribution to conjugation, which we propose is related to the translocation of the TraJ protein to the outer membrane.