Repair replication of DNA in ultraviolet irradiated Mycoplasma laidlawii B

Abstract

Cesium chloride density-gradient techniques were used to detect repair replication in the pleuropneumonia-like organism Mycoplasma laidlawii B following ultraviolet irradiation. Two methods of analysis were used, the first assaying repair in parental DNA shortly after u.v. irradiation, the second assaying repair in the parental DNA only after one round of semi-conservative replication. The second method assays repair only in DNA which has been rendered functional with respect to normal replication following the u.v. irradiation. For a dose of 85 ergs/mm 2, the kinetics of repair replication were linear to apparent saturation after about 0.5 generation and a replacement of 1.2% of the M. laidlawii B chromosome. A dose-response analysis showed that the amount of repair increased rapidly with dose at low u.v. doses (less than 5 ergs/mm 2), but then more slowly to an apparent saturation at about 100 ergs/mm 2. Both assay methods yielded essentially the same values, indicating no difference between the extent of repair replication ahead of and behind the normal replication growing point. Visible light illumination after ultraviolet light reduced the amount of repair replication, as consistent with the existence of an enzymic mechanism for photoreactivation in this organism. The existence of repair replication in the mycoplasmas, possibly the simplest of free-living organisms, demonstrates the importance of repair mechanisms for the maintenance of genetic continuity in living systems.