A WD40-repeat containing protein, similar to a fungal co-repressor, is required for transcriptional gene silencing in Chlamydomonas

In higher plants, mammals, and filamentous fungi, transcriptional gene silencing is frequently associated with DNA methylation. However, recent evidence suggests that certain transgenes can be inactivated by a methylation independent mechanism. In the unicellular green alga Chlamydomonas reinhardtii, single-copy transgenes are transcriptionally silenced without discernible cytosine methylation of the introduced DNA. We have isolated a Chlamydomonas gene, Mut11, which is required for the transcriptional repression of single-copy transgenes. Mut11 appears to have a global role in gene regulation since it also affects transposon mobilization, cellular growth, and sensitivity to DNA damaging agents. In transient expression assays, a fusion protein between the predicted Mut11 gene product (Mut11p) and E. coli β-glucuronidase localizes predominantly to the nucleus. Mut11p, a polypeptide of 370 amino acids containing seven WD40 repeats, is highly homologous to proteins of unknown function that are widely distributed among eukaryotes. Mut11p also shows similarity to the C-terminal domain of TUP1, a global transcriptional co-repressor in fungi. Based on these findings we speculate that, in Chlamydomonas, the silencing of certain single-copy transgenes and dispersed transposons integrated into euchromatic regions may occur by a mechanism(s) similar to those involving global transcriptional repressors. Our results also support the existence, in methylation-competent organisms, of a mechanism(s) of transcriptional (trans)gene silencing that is independent of DNA methylation.