The NC2 alpha and beta subunits play different roles in vivo

NC2 is a heterodimeric regulator of transcription that plays both positive and negative roles in vivo. Here we show that the alpha and beta subunits of yeast NC2 are not always associated in a tight complex. Rather, their association is regulated, in particular by glucose depletion. Indeed, stable NC2 alpha/beta complexes can only be purified from cells after the diauxic shift when glucose has been depleted from the growth medium. In vivo, the presence of NC2 alpha, but not NC2 beta, at promoters generally correlates with the presence of TBP and transcriptional activity. In contrast, increased presence of NC2 beta relative to TBP correlates with transcriptional repression. NC2 is regulated by phosphorylation. We found that mutation of genes encoding casein kinase II (CKII) subunits as well as potential CKII phosphorylation sites in NC2 alpha and beta affected gene repression. Interestingly, NC2-dependent repression in the phosphorylation site mutants was only perturbed in high glucose when NC2 beta and NC2 alpha are not associated, but not after the diauxic shift when NC2 alpha and beta form stable complexes. Thus, the separation of NC2 alpha and beta function indicated by these mutants also supports the existence of multiple NC2 complexes with different functions in transcription.