A histone H1 protein in sea urchins is encoded by a poly(A)⁺ mRNA

Typical histone genes lack intervening sequences and encode small mRNAs (400-800 nucleotides) with short leader and trailer regions. Most histone mRNAs are not polyadenylated but rather terminate in a highly conserved stem and loop structure. The early, late, and testis-specific histone genes of sea urchins, described to date, have this typical histone gene structure. We have identified an unusual H1 gene, H1-δ, in sea urchins that encodes a poly(A)⁺ mRNA. This mRNA is one of a group of polyadenylated transcripts homologous with H1 gene probes. The sequence of H1-δ has been determined. H1-δ encodes a different H1 protein. Although the temporal expression of H1-δ mRNA is similar to that of other late H1 (β and γ) mRNAs, its spatial distribution at the time of maximal accumulation is distinct and confirms that H1-δ is regulated differently than other H1 genes.