Involvement of a recombination repair function in disciplined cell division of Micrococcus radiodurans

When a culture of the temperature-sensitive DNA mutant Micrococcus radiodurans tsI is irradiated with a sublethal dose of ultraviolet or ionizing radiation and is plated immediately, all the bacteria give rise, after 36 h incubation, to colonies identical to those derived from unirradiated bacteria. However, when the irradiated population is held at its restrictive temperature (39 degrees C) (restrictive temperature holding) for 3 h before being plated, less than 0-1% of the surviving bacteria give rise to normal colonies, the rest producing, after incubation for 96 h, small malformed colonies. Qualitatively, the same effect is observed when u.v.-irradiated wild-type M. radiodurans is incubated at 39 degrees C in the presence of nalidixic acid before plating. Compared with the loss of viability, the loss of normal colony development as a function of the radiation dose is sensitive, having I/e values of 210 ergs/mm2 for u.v. radiation and of 4 to 5 krad for 60Co gamma-radiation. These are identical to the radiation dose-response values of a recombination-deficient mutant of M. radiodurans. At first the abnormal colonies consist entirely of giant bacteria but eventually a few bacteria with normal morphology appear and because of their much faster generation time a highly sectored colony results. These colonies can be "rescued" by plating the irradiated bacteria held at 39 degrees C on agar containing pantoyl lactone, their growth being identical to that of unirradiated bacteria. Abnormal colony development is not a general phenomenon in temperature-sensitive mutants of M. radiodurans but occurs in those mutants which are sensitized to radiation when held at 39 degrees C. It is concluded that these abnormal colonies are produced as a result of a defect in a recombination function and that this function is also involved in the regulation of normal cell division.