Yeast L double-stranded ribonucleic acid is synthesized during the G1 phase but not the S phase of the cell cycle.

Abstract

The cytoplasm of Saccharomyces cerevisiae contains two major classes of protein-encapsulated double-stranded ribonucleic acids (dsRNA's), L and M. Replication of L and M dsRNA's was examined in cells arrested in the G1 phase by either alpha-factor, a yeast mating pheromone, or the restrictive temperature for a cell cycle mutant (cdc7). [3H]uracil was added during the arrest periods to cells prelabeled with [14C]uracil, and replication was monitored by determining the ratio of 3H/14C for purified dsRNA's. Like mitochondrial deoxyribonucleic acid, both L and M dsRNA's were synthesized in the G1 arrested cells. The replication of L dsRNA was also examined during the S phase, using cells synchronized in two different ways. Cells containing the cdc7 mutation, treated sequentially with alpha-factor and then the restrictive temperature, enter a synchronous S phase when transferred to permissive temperature. When cells entered the S phase, synthesis of L dsRNA ceased, and little or no synthesis was detected throughout the S phase. Synthesis of L dsRNA was also observed in G1 phase cells isolated from asynchronous cultures by velocity centrifugation. Again, synthesis ceased when cells entered the S phase. These results indicate that L dsRNA replication is under cell cycle control. The control differs from that of mitochondrial deoxyribonucleic acid, which replicates in all phases of the cell cycle, and from that of 2-micron DNA, a multiple-copy plasmid whose replication is confined to the S phase.

Macromolecular syntheses in the cell cycle mutant cdc25 of budding yeast

“…A continued synthesis of ds-killer RNA at restrictive temperature has been reported previously for other G I mutants [18,24]. The degree of inhibition of the rate of synthesis of stable RNA species (about 50%) was much lower than that expected on the basis of the kinetics of stable RNA accumulation during the first 3 -4 h at 36 "C, thus suggesting that a significant degradation of stable R N A also occurs.…”

Conservative replication of double-stranded RNA in Saccharomyces cerevisiae by displacement of progeny single strands.

“…The killer strain 116-1B (a cdc7 [K+ R+]) is essentially congenic with the strain used previously to demonstrate cell cycle control of dsRNA replication (37). RNA was isolated from exponential phase 116-1B cells growing at 23°C.…”

“…In addition, we have not observed any substantial degradation of the rRNA species as judged by ethidium bromide flourescence of the same Northern gels which were to be probed with either L or M [32P]cDNA (data not shown). cell cycle control (37). Cells in the Gl phase of the cell cycle replicate these molecules, and cells arrested in the Gl phase either by ox factor or by a cdc7 mutation accumulate them.…”

“…Neither the mechanism of replication nor the way replication is integrated within the cell cycle is clearly understood. However, in at least one yeast strain, L dsRNA replication is cell cycle phase specific, occurring in Gl phase but not S phase (22,37).Infectious dsRNA viruses are found in vertebrates (reovirus and Colorado tick fever virus), plants (clover wound tumor and rice dwarf virus), and bacteria (Pseudomonas bacteriophage 46) (29). Information concerning the replication of two of these viral dsRNAs is available.…”

“…Neither the mechanism of replication nor the way replication is integrated within the cell cycle is clearly understood. However, in at least one yeast strain, L dsRNA replication is cell cycle phase specific, occurring in Gl phase but not S phase (22,37).…”