No abstract

Sequences at both the 5' and 3' ends of mouse histone genes contribute to the expression of individual genes. The 3' sequences required for high expression of the mouse H2a-614 gene are the same as the sequences required for 3'-end formation. When these sequences were substituted for the 3' end of the poorly expressed H2a-291 gene, expression of the H2a-291 gene was increased fivefold. A 65-nucleotide fragment containing the H2a-614 3' processing signal increased expression of the H2a-291 gene when it was placed in the proper orientation downstream of the H2a-291 3' end. The only mRNAs that accumulated from this gene ended at the H2a-291 3' end, which suggests that the transcript is sequentially processed. In an in vitro processing system, the different histone 3' ends showed different processing efficiencies, which correlated with their expression in cells. These results suggest that the efficiency of processing is important in determining the steady-state levels of individual mouse histone mRNAs.The steady-state concentrations of an mRNA are determined by three factors: (i) rate of transcription, (ii) efficiency of processing and transport to the cytoplasm, and (iii) half-life in the cytoplasm. There are numerous examples of the importance of the rate of transcription and half-life to the steady-state concentrations of an mRNA. Much less is known about the efficiency of RNA processing and transport of RNA to the cytoplasm.The replication-dependent histone genes are unique among eucaryotic genes in that they lack intervening sequences and code for mRNAs that are not polyadenylated. Instead, histone mRNAs end in a stem-loop structure, consisting of a 6-base stem and 4-base loop, which has been highly conserved in evolution (4, 15). The 3' end of histone mRNA is formed in a processing event that requires a U7 small nuclear ribonucleoprotein (snRNP) and at least one other component (10,19,22,23). The levels of histone mRNA are regulated during the cell cycle by multiple mechanisms, including alterations in transcription rate, efficiency of 3'-end formation, and changes in mRNA stability (reviewed in references 21 and 26).In mice, the histone genes are present in two clusters on chromosomes 3 and 13 (20). The individual genes on chromosome 3 account for a much higher proportion of the histone mRNA than do the histone genes on chromosome 13 (13), although both sets of genes are regulated coordinately with DNA synthesis during the cell cycle (8, 13). We have recently shown that sequences both 5' and 3' to the genes on chromosome 3 contribute independently to the high level of expression (17). Here we show that the effect of the 3' sequences is to confer a higher efficiency of processing on the histone transcripts from chromosome 3, suggesting that a substantial portion of the transcripts from the genes on chromosome 13 must be degraded in the nucleus and never reach the cytoplasm. and the histone gene constructs, using calcium phosphate precipitation as described previously (1,6). Chinese hamster ovary (CHO) cells we...