The role of deoxyribonucleic acid (DNA) synthesis in the Escherichia coli conjugation system has been studied using nalidixic acid (NAL) to specifically inhibit DNA synthesis in matings between reciprocal combinations of male (Hfr) and female (F−) mutants resistant and sensitive to NAL; the physiological action of NAL on the strains utilized was also studied. Matings between combinations of mutants resistant (Nalr) and sensitive (Nals) to NAL allow various parental functions to be established: pair formation studies show that the female cells are responsible for the slight decrease in pair formation when NAL is present in Hfr(Nals) × F− (Nals) matings. Preformed mating pairs are stable in the presence of NAL. In matings between Hfr(Nals) and F−(Nalr), the transfer gradient constant increases linearly with low NAL concentration (0.1 to 0.6 μg of NAL per ml). Higher concentrations of NAL (5 μg/ml) act on Nals males to rapidly stop chromosome transfer; under these conditions, however, DNA degradation is unmeasurable as determined from single-strand nicking in the male cells. This result is consistent with a model for chromosome transfer which requires DNA synthesis in the male cell. Inhibition of DNA synthesis (by 85%) in the female has no effect on conjugal chromosome transfer. High concentrations of NAL (>20 μg/ml) produce slight inhibition in chromosome transfer for the Hfr(Nalr) × F−(Nalr) mating tested; this effect is probably caused by action of NAL on the male. The inhibition of chromosomal transfer by NAL appears to be irreversible in the normal sense. A pulse of NAL, applied during transfer, immediately stops the transfer which is in progress. On removal of the NAL block, the temporal appearance of recombinants is consistent with the idea that a new round of transfer has commenced from the sex factor location on the male chromosome
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