No abstract

Although eukaryotic genes are usually transcribed individually, at least a few Caenorhabditis elegans genes appear to be transcribed polycistronically in clusters resembling bacterial operons. The spliced leader SL2 (ref. 2) is specific for trans-splicing to downstream genes in these operons. In addition, many C. elegans pre-mRNAs are trans-spliced to SL1 (ref. 3) near the 5' ends of pre-mRNAs. Because operons have not previously been found in higher eukaryotes, we have investigated how widespread they are in the C. elegans genome. We identified gene clusters using the extensive data generated by the genome project and tested seven for trans-splicing specificity. All were found to fit expectations for polycistronic transcription. In addition, we surveyed reported C. elegans genes for trans-splicing specificity. Both methods indicate that the pre-mRNAs of about 70% of C. elegans genes are trans-spliced and as many as a quarter are transcribed in operons.

The polycomb group (Pc‐G) genes are responsible for maintaining the repressed state of homeotic genes during development. It has been suggested that the Pc‐G exerts its transcriptional control by regulating higher order chromatin structure. In particular, the finding of genetic and molecular similarities to components involved in heterochromatin formation, led to the proposal that homeotic genes are permanently repressed by mechanisms similar to those responsible for heterochromatin compaction. Because of synergistic effects, Pc‐G gene products are thought to act in a multimeric complex. Using immunoprecipitation we show that two members of the Pc‐G, Polycomb and polyhomeotic, are constituents of a soluble multimeric protein complex. Size fractionation indicates that a large portion of the two proteins are found in a distinct complex of molecular weight 2–5 × 10(6) Da. During embryogenesis the two proteins show the same spatial distribution. In addition, by double‐immunofluorescence labelling we can demonstrate that Polycomb and polyhomeotic have exactly the same binding patterns on polytene chromosomes of larval salivary glands. We propose that some Pc‐G proteins act in multimeric complexes to compact the chromatin of stably repressed genes like the homeotic regulators.

The Polycomb group (PcG) genes in Drosophila melanogaster are required for maintenance of correct spatial expression of homeotic genes, and their products are thought to form either a regulatory network or act as a multimeric complex. Recently, it has been suggested that because of homology between Polycomb (Pc) and Su(var)205, PcG genes encode chromatin proteins required for the maintenance of a determined state in chromatin. The polyhomeotic {ph) gene is a member of the PcG of genes. We present DNA sequence of a ph cDNA, which encodes a 169-kD protein with a single putative zinc finger, a serine/threonine-rich region, and has glutamine repeats, suggesting that ph is a DNA-binding protein. Polyclonal antisera directed against ph protein bind to -80 sites on polytene chromosomes. Most of these sites appear to be the same as those recognized by antibodies to Pc protein, ph protein binds to insertion sites of constructs containing DNA from the bithoraxoid {bxd) region of the Bithorax complex, showing that ph binding to chromatin is DNA dependent. The same bxd constructs are recognized by Pc protein, strongly supporting the hypothesis that ph and Pc interact directly.