F- phenocopies: characterization of expression of the F transfer region in stationary phase.

Abstract

The phenomenon of 'F- phenocopies' in which F+ cells become transfer-deficient in stationary phase seems contradictory to the proposed role for F transfer in adaptive mutation during stationary phase induced by nutrient limitation. The expression of a range of transfer genes at the transcriptional and translational level in stationary phase has been characterized as well as the degree of nicking at the origin of transfer, oriT. Transfer efficiency rapidly decreased in mid-exponential phase, coincident with a decrease in traM transcripts. Approximately 2 h later, the transcript for traA, encoding F-pilin, also decreased to undetectable levels. The levels of TraA (pilin), TraD, TraJ and TraT remained fairly constant well into stationary phase while the levels of TraM and Tral decreased to undetectable levels in early stationary phase. A null mutation in the gene for the alternative sigma factor, rpoS, did not affect mating efficiency or transcript levels but did increase the stability of TraM and Tral in stationary phase. Nicking at oriT was detected at maximal levels in early stationary phase and at low levels in late stationary phase. The results suggest that the F-pilus transfer apparatus is maintained in the cell envelope after transcription of the transfer region from the main promoter, Py, has ceased with down-regulation of traM transcription being the first step detected in this process. The presence of a low level of nicking at oriT in stationary phase is consistent with a role for F in promoting adaptive mutation.