Protein Kinase CK2 Holoenzyme Promotes Start-Specific Transcription in Saccharomyces cerevisiae

Abstract

In Saccharomyces cerevisiae, the entrance into S phase requires the activation of a specific burst of transcription, which depends on SBF (SCB binding factor, Swi4/Swi6) and MBF (MCB binding factor, Mbp1/Swi6) complexes. CK2 is a pleiotropic kinase involved in several cellular processes, including the regulation of the cell cycle. CK2 is composed of two catalytic subunits (␣ and ␣=) and two regulatory subunits (␤ and ␤=), both of which are required to form the active holoenzyme. Here we investigate the function of the CK2 holoenzyme in Start-specific transcription. The ckb1⌬ ckb2⌬ mutant strain, bearing deletions of both genes encoding CK2 regulatory subunits, shows a delay of S-phase entrance due to a severe reduction of the expression of SBF-and MBF-dependent genes. This transcriptional defect is caused by an impaired recruitment of Swi6 and Swi4 to G 1 gene promoters. Moreover, CK2 ␣ and ␤= subunits interact with RNA polymerase II, whose binding to G 1 promoters is positively regulated by the CK2 holoenzyme. Collectively, these findings suggest a novel role for the CK2 holoenzyme in the activation of G 1 transcription. In the budding yeast Saccharomyces cerevisiae, the entrance into S phase requires the execution of a series of molecular events in the regulatory area called Start (see references 1 and 2 for recent reviews), in which the cell has to activate a specific burst of transcription of about 200 genes (G 1 regulon) involved in bud emergence, DNA synthesis, and spindle pole body duplication (3). Start-specific transcription depends upon two related transcription factors, SBF and MBF, each of which contains the common transcriptional modulator Swi6 and a DNA binding protein, Swi4 in SBF and Mbp1 in MBF. The SBF complex primarily regulates genes involved in cell morphogenesis, spindle pole body duplication, and other growth-related functions, among which are the CLN1 and CLN2 genes. The MBF complex targets genes involved in DNA replication, DNA repair, and metabolism, such as CLB5, CLB6, and RNR1 (4). However, the separation between SBF-and MBF-controlled expression is not absolute, since many genes are regulated by both SBF and MBF complexes (5).The specific and coherent expression of G 1 -specific genes is achieved by the interaction of SBF and MBF complexes with many different regulators, such as chromatin remodelling factors (Rpd3, Hos3, and the FACT complex), coactivators, and repressors (Stb1 and Whi5) (6-13). The combined actions of all these proteins allow the recruitment of the mediator complex, the RNA polymerase II complex, and several general transcription factors (GTFs), which ultimately leads to transcription initiation. Strikingly, many kinases are known to phosphorylate key components of SBF and MBF, such as Cdk1 (14, 15), Pho85 (8), Rad53 (16,17), Slt2 (18,19), and Hrr25 (20). Recently, we showed that protein kinase Snf1/AMPK (AMP-activated protein kinase), which is a central controller of cellular energy and required for responses to different cellular stresses (21), interacts with Swi...

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