Specific contacts between mammalian U7 snRNA and histone precursor RNA are indispensable for the in vitro 3'' RNA processing reaction.

We have made a detailed molecular analysis of the reactions leading to the formation of mature 3' ends in mammalian histone mRNAs. Using two analytical protocols we have identified an essential sequence motif in the downstream spacer which is consistently present, albeit in diffuse form, mammalian histone genes. Tampering with this sequence element completely abolishes 3' processing. However, 3' cleavage in vitro, although at a very much reduced rate, can be detected when the conserved hairpin is deleted from histone precursor mRNAs. U7 snRNA, previously shown to be essential for the maturation of sea urchin histone messages, was isolated from murine cells and the sequence was determined. The approximately 63-nucleotide, trimethyl-G-capped, murine U7 snRNA possesses a sequence shown in the sea urchin U7 to be required for Sm-precipitability, and like the sea urchin U7, the 3' end of murine U7 is encased in a hairpin structure. The 5' sequence of murine U7 exhibits extensive sequence complementarity to the conserved downstream motif of the histone precursor. As expected, oligo-nucleotide-directed RNase H cleavage of this portion of murine U7 inhibits the in vitro processing reaction. These experiments identify a set of specific contacts between mammalian U7 and histone precursor RNA which is indispensable for the maturation reaction.     

Translation is required for regulation of histone mRNA degradation

When DNA synthesis is inhibited, the mRNAs coding for the replication-dependent histone proteins are selectively destabilized. The histone genes have been altered and reintroduced into tk- mouse L cells by cotransfection with the herpesvirus thymidine kinase gene. Two features of the mRNA are necessary for regulation of degradation: first, the hairpin loop must be present at the 3' end of the histone mRNA; and second, the histone mRNA must be capable of being translated to within 300 nucleotides of the 3' end of the RNA. Polyadenylated histone mRNAs are stable, as are histone mRNAs that contain in-frame termination codons early in the coding region or 500 nucleotide 3' untranslated regions with a normal hairpin loop at the 3' end.     

Structure of a cluster of mouse histone genes.

The four mouse histone genes (2 H3 genes, an H2b gene and an H2a gene) present in a cloned 12.9 kilobase fragment of DNA have been completely sequenced including both 5' and 3' flanking regions. These genes are expressed in cultured mouse cells and the 3' and 5' ends of the mRNA have been determined by S1 nuclease mapping. These genes code for a minor fraction of the histone mRNAs expressed in cultured mouse cells. They comprise at most 5-8% of the total histone mRNA of each type. The two H3 genes code for H3.2 and H3.1 histone proteins, while the H2b gene codes for an H2b.1 protein with a single amino acid change (val-leu) at position 18. Only the 3' portion of the H2a gene is contained in the clone and there is an amino acid change (alanine-proline) at position 126. Comparison of the 5' and 3' flanking sequences reveals a conserved sequence at the 3' end of the mRNA which forms a hairpin loop structure. The codon usage in the genes is non-random and there has been no discrimination against CG doublets in the coding region of the genes.     

A regulatory sequence near the 3'' end of sea urchin histone genes.

The 3' flanking sequences of all five histone genes have been sequenced in the histone DNA clone h19 of the sea urchin Psammechinus miliaris. A large (23 bp) and a small (10 bp) conserved sequence was found by sequence comparison, some 29-40 bp downstream from the termination codon. 12 bases of the larger homology block show a dyad symmetry. The available sequences of clone h22 of the same species and those of the histone clones pSp2 and pSp17 of Strongylocentrotus purpuratus, another sea urchin species, fit well into this comparison. Two types of sequences are involved in the dyad symmetry; one is H1, H3 and H4 specific, the other is H2A and H2B specific. If these conserved sequences are transcribed, a hairpin loop could form in the RNA molecules. This secondary structure might serve as a recognition signal for a regulatory protein.