Abstracts Biological Sciences

Genetic studies on the blue-green alga, Anacystis nidulans

by Yukio Asato

Typescript. Bibliography: leaves [97]-101. viii, 101 l illus Inactivation studies on Anacystis nidulans were performed using UV irradiation and nitrosoguanidine. UV irradiation had no effect when the treated culture was first incubated in the light. When the UV irradiated culture was first incubated in the dark for 9 hours, an exponential survival curve was obtained with slope k=0.23 log S/So ergs•cm. The photoreactivable sector was calculated to be one. A slight shoulder in the survival curve was observed. Extrapolation of the asymptote to the y-axis indicated a target number of 2. The response to nitrosoguanidine was found to be concentration dependant. Mutation experiments were conducted mainly with nitrosoguanidine. A variety of mutant phenotypes was found upon induction with nitrosoguanidine treatment. These were minute colony formers (mcf), filamentous forms (snakes or sna), yellow (yel) and blue (blu) , polymixin B (pmb) and kanamycin (kan) resistance. The frequencies of occurrence were: 3.7x10^-3 (mcf), 1.0x10^-3 (sna) , 1.8x10^-3 (yel), 2.98x10^-3 (blu) and 4.8x10^-4 (pmbr). Auxotrophic mutants were not recovered even after screening approximately 8.0x10^4 surviving colonies with the replica plate method. Cell synchrony was induced by a light-dark regimen. The rates of macromolecular synthesis of the synchronous culture were determined. Protein, RNA, and phospholipid fractions increased exponentially. DNA synthesis was periodic. An abrupt increase in the rate of DNA synthesis was seen at 3 hours prior to the cell division periods. The period of DNA synthesis was 6 to 7 hours. The times of abrupt increases in the rate of DNA synthesis were inferred to be the times of initiation of genome at a fixed point of replication. A genetic map was constructed by taking advantage of cyclic DNA synthesis of synchronously dividing cells. Sequential mutagenesis during the period of DNA synthesis yielded unique temporal patterns for specific marker frequencies. The patterns showed an initial plateau, an abrupt increase, then, a second plateau. Slopes of the abrupt increases were calculated. The times of the abrupt increases were then designated as the times of the replication of the cistrons and, correspondingly, the loci of the markers in time units. The loci of six markers were presented as the genetic map of Anacystis nidulans. These were pmbr , blu, yel, sna, mcf, and kanr in that temporal order.