F- PHENOCOPIES - CHARACTERIZATION OF EXPRESSION OF THE F-TRANSFER REGION IN STATIONARY-PHASE

The phenomenon of 'F- phenocopies' in which F+ cells become transfer-d eficient 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 t he transcriptional and translational level in stationary phase has bee n characterized as well as the degree of nicking at the origin of tran sfer, oriT. Transfer efficiency rapidly decreased in mid-exponential p hase, 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 r emained fairly constant well into stationary phase while the levels of TraM and Tral decreased to undetectable levels in early stationary ph ase. A null mutation in the gene for the alternative sigma factor, rpo S, did not affect mating efficiency or transcript levels but did incre ase the stability of TraM and Tral in stationary phase. Nicking at ori T 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 transcript ion of the transfer region from the main promoter, Py, has ceased with down-regulation of traM transcription being the first step detected i n this process. The presence of a low level of nicking at oriT in stat ionary phase is consistent with a role for F in promoting adaptive mut ation.