The role of gyrA, gyrB, and dnaA functions in bacterial conjugation

The role of DNA gyrase in F'lac plasmid conjugation was studied using Escherichia coli gyrA43 (Ts), gyrB41(Ts), and dnaA46(Ts) thermosensitive mutants as donor or recipient organisms, and a rifampicin or nalidixic acid-resistant J-53 strain in the presence or absence of nalidixic acid. Mating experiments were also performed employing Hfr derivatives of the thermosensitive strains. Conjugation was carried out in broth for 60 min using a standard method at permissive and non-permissive (32 and 43 A degrees C) temperatures, with or without drugs. At 32 A degrees C, nalidixic acid reduced the number of transconjugants by about 97 % in comparison to the control, while at 43 A degrees C, the drug inhibited F'lac transfer by about 98 % from dnaA46(Ts) mutant and by about 6.5 % from gyrA43(Ts) and 15 % from gyrB41(Ts) hosts. Using the temperature-sensitive mutants as recipient strains, the transconjugants found were approximately the same under all conditions. The number of transconjugants did not change significantly when nalidixic acid-resistant strains were used as donor or recipient strains. Lastly, nalidixic acid reduced the number of transconjugants from Hfr selected in the above mutants under all experimental conditions. These findings suggest that F'lac transfer does not involve DNA gyrase activity.