Molecular Biology

A Fourth Musketeer

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Science  04 Mar 2005:
Vol. 307, Issue 5714, pp. 1377
DOI: 10.1126/science.307.5714.1377c

In eukaryotic cells, the enzymatic activities of RNA polymerases I, II, and III produce ribosomal RNA (rRNA), messenger RNA, and transfer RNA (and 5S rRNA), respectively. However, the genome sequence of Arabidopsis thaliana revealed that another RNA polymerase might exist, and Onodera et al. provide evidence for a functional RNA polymerase IV (Pol IV). Mutant plants lacking RPD1 and RPD2, genes encoding the two largest subunits of the putative Pol IV, were still viable, but higher order heterochromatin assembly into centromeres was disrupted. Generally, an increase in cytosine methylation favors the formation of condensed heterochromatin. In rpd2 plants, cytosine methylation of the pericentromeric 5S rRNA gene clusters was low, and these clusters did not cycle from a decondensed transcriptionally active state into inactive heterochromatin. Because small interfering RNAs (siRNAs) complementary to 5S rRNA genes were also reduced, the authors suggest that Pol IV affects amplification of siRNAs that direct DNA methylation (of their corresponding genes) and hence promote the organization of condensed nuclear chromocenters. — LDC

Cell 10.1016/S0092867405001510 (2005).

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