ABSTRACT
Two nonallelic pairs of late H2A and H2B genes of the sea urchin Psammechinus miliaris were isolated on two different cosmid clones. The genes of cosmid PmL1 are separated by 11 kilobases of DNA and code for the late H2A-2 and H2B-2 variants. The genes of clone PmL2 are divergently transcribed with 1,060 base pairs of intergenic spacer DNA and code for novel variants of the H2A-2 and H2B-2 type. A comparison of the promoter sequences revealed little homology upstream of the TATA box with the exception of a 24-base-pair-long conserved sequence which is present at the same position in both late H2B promoters and part of which is identical with the "H2B-specific" 5' element. The mRNAs of the H2A and H2B genes of cosmid PmL1 reach their maximal levels early in the mesenchyme blastula embryo, whereas the transcripts of both genes of clone PmL2 accumulate maximally only later in the pluteus larva. In the adult sea urchin all four mRNAs are present in the tube foot but not in the intestine and lantern muscle. This pattern of differential expression in the embryo and tissue-specific expression in the adult suggests cell lineage-specific regulation of the late H2A-2 and H2B-2 genes. Another class of late histone genes represented by the H2A-3 and H2B-1 genes was shown to be expressed in all three adult tissues tested, whereas transcripts of the late H2A-1 genes could not be detected, suggesting that these genes are active exclusively during sea urchin development.
