No abstract

The properties of heterochromatin are reconsidered in the context of our present understanding of gene silencing, telomeric and centromeric properties, position-effect variegation and X-chromosome inactivation. It is proposed that the chromatin in heterochromatic chromosomal regions is generally similar in its molecular composition to that in silenced chromosomal regions. Heterochromatic appearance hence reflects not a particular quality of the respective chromosomal regions but only a specific kind of chromatin packaging comparable to that required for the inactivation of genes. This packaging may be initiated by particular signals in the DNA but can be propagated over more extended chromosomal regions by the formation of multiprotein complexes that interact with histones and possibly cell-specific additional components (RNA or proteins) that determine the status of the chromosome in a particular cell type.

We demonstrate that in Drosophila melanogaster the histone H3.3 replacement variant is encoded by two genes, H3.3A and H3.3B. We have isolated cDNA clones for H3.3A and cDNA and genomic clones for H3.3B. The genes encode exactly the same protein but are widely divergent in their untranslated regions (UTR). Both genes are expressed in embryos and adults; they are expressed in the gonads as well as in somatic tissues of the flies. However, only one of them, H3.3A, shows strong testes expression. The 3' UTR of the H3.3A gene is relatively short (approximately 250 nucleotides (nt)). H3.3B transcripts can be processed at several polyadenylation sites, the longest with a 3' UTR of more than 1500 nt. The 3' processing sites, preferentially used in the gonads and somatic tissues, are different. We have also isolated the Drosophila hydei homologues of the two H3.3 genes. They are quite similar to the D. melanogaster genes in their expression patterns. However, in contrast to their vertebrate counterparts, which are highly conserved in their noncoding regions, the Drosophila genes display only limited sequence similarity in these regions.