Direct Regulation of tRNA and 5S rRNA Gene Transcription by Polo-like Kinase 1

Polo-like kinase Plk1 controls numerous aspects of cell-cycle progression. We show that it associates with tRNA and 5S rRNA genes and regulates their transcription by RNA polymerase Ill (pol Ill) through direct binding and phosphorylation of transcription factor Brit During interphase, Plk1 promotes tRNA and 5S rRNA expression by phosphorylating Plk1 directly on serine 450. However, this stimulatory modification is overridden at mitosis, when elevated Plk1 activity causes Bill phosphorylation on threonine 270 (T270), which prevents pol III recruitment. Thus, although Plk1 enhances net tRNA and 5S rRNA production, consistent with its proliferation-stimulating function, it also suppresses untimely transcription when cells divide. Genomic instability is apparent in cells with BM T270 mutated to alanine to resist Plk1 -directed inactivation, suggesting that chromosome segregation is vulnerable to inappropriate pol III activity.