MOLECULAR CHARACTERIZATION OF HISTONE GENES OF THE NEMATODE CAENORHABDITIS ELEGANS AND THE SEA URCHIN LYTECHINUS PICTUS

Abstract

Histones are a family of basic proteins that are fundamental in the structural organization of chromatin. Genes that encode histone proteins form multigene families in most eukaryotes. This study focusses on the organization and structural characteristics of histone genes of the nematode Caenorhabditis elegans and the sea urchin Lytechinus pictus. The core histone genes (H2A, H2B, H3, H4) of C. elegans are organized in approximately 12 dispersed clusters. Genomic clones which contain histones have at least one copy of each core histone gene per cluster. Analysis of cosmid clones which overlap three different clusters shows that no other histone clusters are closer than 8 to 60 thousand basepairs. Gene order within the histone clusters varies, and histone mRNAs are transcribed from both DNA strands. The characterized histone clones do not contain histone H1 genes. The organization of histone genes in C. elegans resembles the clustering of histone genes in most vertebrate organisms and in the late stage specific histone gene family of sea urchin. Nucleotide sequence analysis of histone genes from two different clusters indicates that C. elegans core histone genes do not contain intervening sequences or poly(A) addition signals. A sequence which is conserved at the 3' end of the C. elegans core histone genes that were analyzed contains the conserved dyad found in histone genes from other multicellular organisms. A sequence which is conserved at the 5' end of each C. elegans core histone gene that was analyzed is not found in the histone genes of other organisms or in other nonhistone genes from C. elegans. The inferred amino acid sequences of C. elegans core histones are homologous to histone proteins of other eukaryotes.;The nucleotide sequences of sea urchin (L. pictus) late stage specific H3 and H4 histone genes were compared with other L. pictus late and early stage histone H3 and H4 genes. Members of each histone gene family are more homologous to each other than to members of the other histone gene family. The spacer regions between two late H3-H4 gene pairs are not homologous; however, a sequence 5'CTCATGTATT3' is conserved upstream of both late H4 genes and another 5'AGATTCA3' is conserved upstream of both H3 genes. The divergence of the spacer region contrasts with the 95-96% conservation found between late histone gene coding sequences. The results suggest that there is intergenic exchange in the germline among members of the late histone gene family and that the unit of exchange is the individual gene rather than the heterotypic dimer which includes the common spacer DNA.