Transcriptional regulators of the Schizosaccharomyces pombe fbp1 gene include two redundant Tup1p-like corepressors and the CCAAT binding factor activation complex

The Schizosaccharomyces pombe fbp1 gene, which encodes fructose-1,6-bis-pho sphatase, is transcriptionally repressed by glucose through the activation of the cAMP-dependent protein kinase A (PKA) and transcriptionally activate d by glucose starvation through the activation of a mitogen-activated prote in kinase (MAPK). To identify transcriptional regulators acting downstream from or in parallel to PIU, we screened an adh-driven cDNA plasmid library for genes that increase fbp1 transcription in a strain with elevated PKA ac tivity. Two such clones express amino-terminally truncated forms of the S. pombe tup12 protein that resembles the Saccharomyces cerevisiae Tup1p globa l corepressor. These clones appear to act as dominant negative alleles. Del etion of both tup12 and the closely related tup11 gene causes a 100-fold in crease in fbp1-lacZ expression, indicating that tup11 and tup12 are redunda nt negative regulators of fbp1 transcription. In strains lacking tup11 and tup12 the atfl-pcrl transcriptional activator continues to play a central r ole in fbp1-lacZ expression; however, spcl MAPK phosphorylation of atfl is no longer essential for its activation. We discuss possible models for the role of tup11- and tup12-mediated repression with respect to signaling from the MAPK and PKA pathways. A third clone identified in our screen expresse s the php5 protein subunit of the CCAAT-binding factor (CBF). Deletion of p hp5 reduces fbp1 expression under both repressed and derepressed conditions . The CBF appears to act in parallel to atfl-pcrl, although it is unclear w hether or not CBF activity is regulated by PKA.