Worm Breeder's Gazette 8(3): 64
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
We have analyzed the arrangement of histone genes on cloned C. elegans restriction fragments by constructing detailed restriction maps and by DNA sequencing. We have studied three cloned segments, each carrying a cluster of histone genes. The clusters are surrounded by non-histone DNA and hence give no indication as yet of a major histone locus of multiple, linked clusters. Two of the clusters carry a single copy of each of the core histone genes arranged in the order H4, H3, H2A and H2B. The orientation of transcription of the H3 gene is opposite to that of the other three. In the third cluster there are two copies of each of the core histone genes, arranged as a duplication of a cluster, with genes in the order H3, H4, H2B, H2A. The duplicate clusters are very similar at the nucleotide level. We calculate from restriction site differences that the sequences are 96% conserved, indicating either that the duplication is of fairly recent evolutionary origin, or has undergone a recent correction event. So far, we have not been able to locate H1 genes on our clones. Heterologous H1 probes from chick and sea urchin do not cross- hybridize with C. elegans genomic sequences, so we do not as yet know the number or arrangement of H1 genes. Northern hybridization studies using the cloned C. elegans clusters as probes have revealed four discrete transcripts of sizes expected for the four core histone genes. The absence of a larger transcript of a size expected for H1 suggests that genes for H1 are not present in the cloned clusters and hence must lie elsewhere in the genome. Sequencing of the entire H2A and H4 genes as well as most of the H3 gene in one cluster reveals near identity with sea urchin proteins. Like most histone genes, these sequences contain no intervening sequences and no AAUAAA polyadenylation signal. Interestingly, C. elegans histone genes also do not contain the conserved 3' mRNA processing site found in histone genes of human, chicken, Drosophila, sea urchin and several other organisms. We therefore believe that C. elegans must utilize a different sequence or mechanism for generating the 3' ends of mature histone messages.