Polyadenylation and U7 snRNP-mediated cleavage: alternative modes of RNA 3' processing in two avian histone H1 genes.

The six chicken histone H1 genes have 3'-processing sequences typical of replication-dependent histone genes, which are expressed as poly(A)- mRNAs. However, by Northern analysis of RNA from several adult chicken tissues, as well as from embryonal skeletal muscle in vivo and in vitro, we have observed histone H1 transcripts longer than those predicted on the basis of the published genomic sequences. These RNAs are polyadenylated transcripts of the genes H1.01 and H1.10, which encode the 'c fraction' H1 protein subtypes. Both transcripts contain an internal stem-loop and purine-rich box associated with the 3' processing of poly(A)- histone mRNAs. The 2-kb poly(A)+ H1.01 transcript is present at high steady-state levels in tissues with low rates of DNA synthesis, has a longer half-life than the poly(A)- mRNA from the same gene, and is polyribosomal in embryonal skeletal muscle. The 1-kb poly(A)+ H1.10 RNA is the major H1.10 transcript in adult skeletal muscle. The properties of these RNAs suggest that they may contribute to the relaxed replication dependence of c fraction subtype expression. The polyadenylation signals of both genes are unusual in their association with processed (nonhistone) pseudogene-like elements, an arrangement with possible implications for the mechanism of alternative 3'-end formation in these genes.