Amplification of sex repressor function of one fi-+ R-factor during anaerobic growth of Escherichia coli.

It has been reported by Mitsuhashi (1965) that transfer of one R-factor was completely inhibited by anaerobic transfer conditions. In contrast, several workers have observed R-factor transfer, although at a reduced rate, in the animal intestines, a largely anaerobic environment. It is shown here that in vitro transfer of the R-factor R1 (F-type pilus, fi-+) in Escherichia coli K-12 is severely depressed, whereas transfer of R64 (I-type pilus, fi-minus) is slightly stimulated by anaerobiosis. Inhibition of R1 fertility is dependent on anaerobic conditions during pregrowth of the donor cells, whereas the oxygen tension during recipient pregrowth, transfer, and plating is of little importance. Anaerobic pregrowth has a less inhibitory effect on the fertility of R1drd19, a mutant of R1 having a defective sex repressor. The fi-+ property of R1 when introduced into F' or Hfr bacteria is amplified during anaerobic growth. These observations strongly indicate that the sex repressor is the mediator of the anaerobic fertility inhibition of the R-factor R1. This hypothesis was supported by studies of the formation of sex pili, the only gene product identified that is controlled by the sex repressor of R1. Using propagation of the F-type pilus-specific phage MS2 as a measure of the degree of sex piliation of a bacterial population, it is shown that in anaerobic cultures sex piliation due to R1 is strongly repressed, whereas piliation due to R1drd19 is repressed to a lesser extent. The possible survival value of the response of R1 towards oxygen tension is discussed.