The levels of histone mRNA increase 35-fold as selectively detached mitotic CHO cells progress from mitosis through G1 and into S phase. Using an exogenous gene with a histone 3' end which is not sensitive to transcriptional or half-life regulation, we show that 3' processing is regulated as cells progress from G1 to S phase. The half-life of histone mRNA is similar in G1-and S-phase cells, as measured after inhibition of transcription by actinomycin D (dactinomycin) or indirectly after stabilization by the protein synthesis inhibitor cycloheximide. Taken together, these results suggest that the change in histone mRNA levels between G1-and S-phase cells must be due to an increase in the rate of biosynthesis, a combination of changes in transcription rate and processing efficiency. In G2 phase, there is a rapid 35-fold decrease in the histone mRNA concentration which our results suggest is due primarily to an altered stability of histone mRNA. These results are consistent with a model for cell cycle regulation of histone mRNA levels in which the effects on both RNA 3' processing and transcription, rather than alterations in mRNA stability, are the major mechanisms by which low histone mRNA levels are maintained during G1.Histone proteins are synthesized coordinately with DNA, and changes in histone protein synthesis are mediated by rapid changes in histone mRNA concentration (10, 13, 32). There are two major groups of histone genes: (i) those coding for the cell cycle-regulated, replication-dependent histones and (ii) the constitutively expressed replacement variant histones (39, 40). The replication-dependent histone genes lack intervening sequences, and the mRNAs end in a 3' stem-loop structure which is formed by an endonucleolytic cleavage (9, 22). The replacement variant histone H3.3 gene contains intervening sequences, and the mRNA ends in a 3' poly(A) sequence (6,38).Detailed studies of the changes in histone mRNA metabolism during the cell cycle have largely been conducted in cells synchronized by using inhibitors of DNA synthesis (13, 26), serum starvation (8), or temperature-sensitive mutants (2,3,18,19,33). Although in one study centrifugal elutriation (1) was used to avoid the effects of drug-induced synchrony, the lack of complete synchronization did not allow precise measurement of the changes in histone mRNA levels during the cell cycle. These studies all showed that the rate of histone gene transcription varied only three-to fivefold during the cell cycle, indicating that much of the regulation must be posttranscriptional.Posttranscriptional regulation at two steps, mRNA degradation and 3' processing of pre-mRNA, has been implicated in control of histone mRNA levels in a number of studies. of increased degradation (4, 10, 32). The 3' stem-loop structure is required for their rapid degradation (11,15,24), and this process also requires that the mRNA be actively translated (11). Inhibitors of protein synthesis prevent the rapid degradation of histone mRNA (4, 35). The replacement variant histone mRNA...
