The enhancer of trithorax and polycomb corto interacts with cyclin G in Drosophila

Background

Polycomb (PcG) and trithorax (trxG) genes encode proteins involved in the maintenance of gene expression patterns, notably Hox genes, throughout development. PcG proteins are required for long-term gene repression whereas TrxG proteins are positive regulators that counteract PcG action. PcG and TrxG proteins form large complexes that bind chromatin at overlapping sites called Polycomb and Trithorax Response Elements (PRE/TRE). A third class of proteins, so-called “Enhancers of Trithorax and Polycomb” (ETP), interacts with either complexes, behaving sometimes as repressors and sometimes as activators. The role of ETP proteins is largely unknown.

Methodology/Principal Findings

In a two-hybrid screen, we identified Cyclin G (CycG) as a partner of the Drosophila ETP Corto. Inactivation of CycG by RNA interference highlights its essential role during development. We show here that Corto and CycG directly interact and bind to each other in embryos and S2 cells. Moreover, CycG is targeted to polytene chromosomes where it co-localizes at multiple sites with Corto and with the PcG factor Polyhomeotic (PH). We observed that corto is involved in maintaining Abd-B repression outside its normal expression domain in embryos. This could be achieved by association between Corto and CycG since both proteins bind the regulatory element iab-7 PRE and the promoter of the Abd-B gene.

Conclusions/Significance

Our results suggest that CycG could regulate the activity of Corto at chromatin and thus be involved in changing Corto from an Enhancer of TrxG into an Enhancer of PcG.     

Transgenic tobacco plants expressing the Drosophila Polycomb (Pc) chromodomain show developmental alterations: possible role of Pc chromodomain proteins in chromatin-mediated gene regulation in plants.

The chromodomain of the Drosophila Polycomb (Pc) protein has been introduced into tobacco nuclei to determine its location in the nucleus and its effect on plant development. Pc is a repressor of homeotic Drosophila genes that shares a well-conserved, although not identical, chromodomain with a structural heterochromatin component, Heterochromatin Protein 1. The chromodomains might therefore play a common role in chromatin repression. An analysis of transgenic plants expressing the Pc chromodomain, which was linked to the green fluorescent protein, suggested that the Pc chromodomain has distinct target regions in the plant genome. Transgenic plants expressing the Pc chromodomain had phenotypic abnormalities in their leaves and flowers, indicating a disruption in development. In axillary shoot buds of plants displaying altered leaf phenotypes, enhanced expression of a homeodomain gene, which is downregulated in wild-type leaves, was found. In Drosophila, Pc has been shown to possess distinct chromosome binding activity and to be involved in the regulation of development-specific genes. Our results support the assumptions that the heterologous chromodomain affects related functions in Drosophila and in plants, and that chromatin modification mechanisms are involved in the regulation of certain plant genes, in a manner similar to chromatin-mediated gene regulation in Drosophila.     

Mammalian chromodomain proteins: their role in genome organisation and expression

The chromodomain is a highly conserved sequence motif that has been identified in a variety of animal and plant species. In mammals, chromodomain proteins appear to be either structural components of large macromolecular chromatin complexes or proteins involved in remodelling chromatin structure. Recent work has suggested that apart from a role in regulating gene activity, chromodomain proteins may also play roles in genome organisation. This article reviews progress made in characterising mammalian chromodomain proteins and emphasises their emerging role in the regulation of gene expression and genome organisation. BioEssays 22:124-137, 2000.     

Arabidopsis L10 ribosomal proteins in UV-B responses

Ribosomal protein L10 (RPL10) is a ubiquitous protein that participates in joining the 40S and 60S ribosomal subunits into a functional 80S ribosome; however, increasing evidence indicates that RPL10 from various organisms has multiple extra-ribosomal functions, besides being a constituent of ribosome and its role in translation. Arabidopsis thaliana contains in its genome three genes encoding RPL10, named RPL10A, RPL10B and RPL10C. Previously, we found that in maize and in A. thaliana, UV-B induces a reduction in protein biosynthesis, probably as a consequence of ribosomal damage; however, cellular recovery occurs in the absence of UV-B. Here, we show that RPL10s are differentially regulated by UV-B in a dosage and time dependent manner: RPL10C is induced, RPL10B is downregulated at high UV-B intensity and RPL10A is not UV-B regulated. In addition, by co-immunoprecipitation studies using RPL10 antibodies and proteins from control and UV-B irradiated Arabidopsis plants, we demonstrate that RPL10 associates with different proteins under the two different conditions, including nuclear proteins, suggesting that at least one isoform may have extra-ribosomal roles.Key words: UV-B exposure, translation, ribosomal protein, co-immunoprecipitation