Getting Repetitive in Keystone

Repeats appear important in gene silencing. Repeats appear important in gene silencing. At least two talks at the Keystone Symposium Conference on Epigenetics and Chromatin Remodeling in Development implicate the power of tandem repeats in RNA-interference induced silencing. Rob Martienssen of Cold Spring Harbor talked about silencing of transposable elements (in keeping with the grand history of his institution). He explained how his lab has found that RNAi-independent and RNAi-dependent mechanisms are required to silence different TEs, and how histone modification differs accordingly. Recapping an idea he had put forth a while back he talked about how tandem repeats appear much more effective in RNAi mediated silencing. The reason, it seems, is that long tandemly repeated RNA transcripts destined for dicer could provide a much bigger pool of short interfering RNAs (siRNAs) and so have better likelihood of hitting the right complementarity for silencing genes at the transcriptional level. This was on work he?d done in __Arabidopsis__, and thus far, for transcriptional silencing via RNAi (a new and increasingly hot topic for discussion, at this meeting) plants seem to serve as the strongest examples. I talked with Vicki Chandler of the University of Arizona after the evening sessions tonight. She?s been showing how tandem repeats are responsible for a so-called paramutation that changes normally dark-blue maize to green. The B-I allele, which when expressed produces the dark coloration, is dominant over most other B alleles save B?. In 100% of B-I-to-B? crosses, the F1 generation shows the B? phenotype. B-I can also spontaneously switch on its own and can then effectively switch other B-I alleles to B? when crossed. Lots of paramutation events have been characterized, but this one is notable for its penetrance and stability. Strange thing is the sequence of the two alleles is exactly the same and differs from other alleles in part due to 7 tandemly repeated sequences 100kb upstream of the gene. The story goes that for some reason B? expresses these tandem repeats and RNAi-mediated transcriptional silencing stably and heritably silences the gene. Reducing the number of repeats reduced the efficiency of the paramutation. So, the more repeats the more effective the silencing.So repeats appear important in silencing. And as Chandler and I talked over a plate of (aptly enough) blue corn tortilla chips, she speculated that the concept may even be a factor in a fly epigenetics story presented this evening by Giacomo Cavalli. Cavalli from CNRS talked about polycomb-group response elements (PREs), genetic regulatory elements through which the powerful developmental regulators in the polycomb and trithorax groups regulate specific genes. Using the PRE Fab7 in front of a reporter silenced the reporter, but not completely. Adding additional copies of Fab7 produced much stronger repression. And RNAi seems to be at work. Some dicer and argonaute mutations that hobble RNAi machinery derepressed Fab7-dependent silencing. Although the repeats aren?t tandem, they do co-localize in the nucleus. Is repetition of the Fab7 sequence adding to the pool of possible siRNAs? I?ll have to ask Cavalli tomorrow.

Getting Repetitive in Keystone

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