Controlling the Masses

Abstract

MOLECULAR BIOLOGY The regulation of gene transcription is complex and occurs at many different stages, of which one is the transformation of chromatin from an open and more active structure to a closed and inactive state. This regulatory challenge is amplified when the target genes are present in hundreds of copies on several different chromosomes. Ribosomes are composed of proteins and RNAs, and the number of genes encoding ribosomal RNAs (rRNAs) ranges from several hundred in mammals to several thousand in plants. They are organized in tandem repeats that reside in nucleolar organizing regions on chromatin, and the activity of these rRNA genes is finely tuned to support cell growth. However, not all rRNA genes are actively transcribed at the same time, and Sanij et al. show that decreasing the amount of upstream binding factor (UBF) causes methylation-independent and reversible silencing of rRNA genes. UBF depletion led to chromatin remodeling, although total rRNA production was only partially reduced, as there was an increase in transcription from the remaining active genes. The authors found that during differentiation, the active pool of rRNA genes actually decreases, along with the levels of UBF, suggesting that UBF may regulate the ratio of active versus inactive rRNA genes during development. — HP [*][1] J. Cell Biol. 183 , 1259 (2008). [1]: #fn-1