REGULATION OF PHO4 NUCLEAR-LOCALIZATION BY THE PHO80-PHO85 CYCLIN-CDKCOMPLEX

PHO4, a transcription factor required for induction of the PHO5 gene i n response to phosphate starvation, is phosphorylated by the PHO80-PHO 85 cyclin-CDK (cyclin-dependent kinase) complex when yeast are grown i n phosphate-rich medium. PHO4 was shown to be concentrated in the nucl eus when yeast were starved for phosphate and was predominantly cytopl asmic when yeast were grown in phosphate-rich medium. The sites of pho sphorylation on PHO4 were identified, and phosphorylation was shown to be required for full repression of PHO5 transcription when yeast were grown in high phosphate. Thus, phosphorylation of PHO4 by PHO80-PHO85 turns off PHO5 transcription by regulating the nuclear localization o f PHO4.