Condensin goes with the family but not with the flow

Extra-transcriptional functions of RNA Polymerase III complexes: TFIIIC as a potential global chromatin bookmark

RNA polymerase III (Pol III) is one of three eukaryotic transcription complexes, and was identified as the complex responsible for production of transfer RNA and a limited number of other small RNAs. Pol III transcription at tRNA genes (tDNAs) requires the binding of two transcription factor complexes, TFIIIC and TFIIIB. Recent evidence points to a larger role for the Pol III transcription system in various other nuclear processes, including effects on nucleosome positioning, global genome and sub-nuclear organization, and direct effects on RNA polymerase II (Pol II) transcription. These effects are perhaps mediated by recruitment of a host of other chromatin proteins, including Pol II transcription factors and chromatin enzymes. Extra-TFIIIC sites (ETC sites) are chromosomal locations bound by TFIIIC without the rest of the Pol III complex, and bound TFIIIC alone is also able to mediate additional functions. These so called "extra-transcriptional effects" of the Pol III system are reviewed here, and a model is put forth suggesting that the TFIIIC transcription factor may act as a stably bound, global "bookmark" within chromatin to establish, maintain, or demarcate chromatin states as cells divide or change gene expression patterns.

Centromeric localization of dispersed Pol III genes in fission yeast

The eukaryotic genome is a complex three-dimensional entity residing in the nucleus. We present evidence that Pol III-transcribed genes such as tRNA and 5S rRNA genes can localize to centromeres and contribute to a global genome organization. Furthermore, we find that ectopic insertion of Pol III genes into a non-Pol III gene locus results in the centromeric localization of the locus. We show that the centromeric localization of Pol III genes is mediated by condensin, which interacts with the Pol III transcription machinery, and that transcription levels of the Pol III genes are negatively correlated with the centromeric localization of Pol III genes. This centromeric localization of Pol III genes initially observed in interphase becomes prominent during mitosis, when chromosomes are condensed. Remarkably, defective mitotic chromosome condensation by a condensin mutation, cut3-477, which reduces the centromeric localization of Pol III genes, is suppressed by a mutation in the sfc3 gene encoding the Pol III transcription factor TFIIIC subunit, sfc3-1. The sfc3-1 mutation promotes the centromeric localization of Pol III genes. Our study suggests there are functional links between the process of the centromeric localization of dispersed Pol III genes, their transcription, and the assembly of condensed mitotic chromosomes.