Cell-cycle regulation of yeast histone mRNA

The levels of H2A and H2B mRNAs as a function of cell-cycle stage were determined by hybridization methods. The analysis was extended to H3 and H4 mRNAs by in vitro translation. Cells were partitioned into cell-cycle stages either by centrifugal elutriation or by G1 synchronization with the yeast mating pheromone, alpha factor. The data lead to the following conclusions. First, histone mRNA can be detected in significant quantities only in S-phase cells. Second, the point of maximal accumulation of histone mRNA is not coincident with the point of maximal DNA synthesis; rather, histone mRNA begins accumulating very early in S, reaching a maximum when less than one half of the DNA has replicated. From this point in the cell cycle the histone mRNA levels decrease, reaching basal levels at the end of S. Third, in spite of the fact that the rate of histone mRNA accumulation is not coincident with the rate of DNA synthesis, the two processes are coupled; inhibition of DNA synthesis results in an extremely rapid disappearance of histone mRNA that is much shorter than the normal histone mRNA half-life. Fourth, there is no visible accumulation of mRNA precursors at any cell-cycle stage. We can conclude that, in yeast, histone mRNA levels are tightly and coordinately regulated throughout cell division and that this regulation most likely occurs at both transcriptional and posttranscriptional levels. We also show that the two genetically unlinked H2B genes present in yeast are both expressed at comparable levels and are regulated. The regulation is probably sequence-specific, since genes in close proximity to the histones are not subject to cell-cycle control.