Stem-loop binding protein, the protein that binds the 3' end of histone mRNA, is cell cycle regulated by both translational and posttranslational mechanisms

The expression of the replication-dependent histone mRNAs is tightly regulated during the cell cycle. As cells progress from G₁ to S phase, histone mRNA levels increase 35-fold, and they decrease again during G₂ phase. Replication-dependent histone mRNAs are the only metazoan mRNAs that lack polyadenylated tails, ending instead in a conserved stem-loop. Much of the cell cycle regulation is posttranscriptional and is mediated by the 3' stem-loop. A 31-kDa stem-loop binding protein (SLBP) binds the 3' end of histone mRNA. The SLBP is necessary for pre-mRNA processing and accompanies the histone mRNA to the cytoplasm, where it is a component of the histone messenger RNP. We used synchronous CHO cells selected by mitotic shakeoff and HeLa cells synchronized at the G₁/S or the M/G₁ boundary to study the regulation of SLBP during the cell cycle. In each system the amount of SLBP is regulated during the cell cycle, increasing 10- to 20-fold in the late G₁ and then decreasing in the S/G₂ border. SLBP mRNA levels are constant during the cell cycle. SLBP is regulated at the level of translation as cells progress from G₁ to S phase, and the protein is rapidly degraded as they progress into G₂. Regulation of SLBP may account for the posttranscriptional component of the cell cycle regulation of histone mRNA.