Organization and complete nucleotide sequence of the core-histone-gene cluster of the annelid Platynereis dumerilii

The arrangement of the core-histone genes, their transcriptional polarity and their nucleotide sequences have been determined for the polychaete annelid Platynereis dumerili. A clone containing the core-histone genes was isolated from a annelid genomic library constructed in the EMBL-4 phage vector, using a trout H3 genomic probe. This clone was found to contain two and a half repeats of a 6-kbp EcoRV fragment that contained one copy of each of the core-histone genes. The clusters are tandemly arrayed in the genome and the gene order within the core-histone cluster does not vary. Absolutely no differences were found in the nucleotide sequences comprising the same part of two adjacent clusters (bases -225 to 2776 and bases 5821 to 8825). The number of copies of the cluster appeared to be high: approximately 660 copies/diploid cell, as also observed in sea urchins and amphibians. There are also some additional subtypes of histone gene organization: multimers of tandemly arrayed genes and isolated genes; these are present at a much lower copy number (an average of 40-50 copies/diploid genome). Two mRNAs (for H2B and H3) are transcribed from one DNA strand and the two other histone mRNAs (for H2A and H4) from the other strand as is the case for some insects and certain vertebrates. No H1-coding sequence has been found in the completely sequenced four-membered cluster. The organization of histone genes in P. dumerilii is similar to the clustering found in Caenorhabditis elegans but in this nematode worm several different types of organization are observed with a low copy number for each.