Dal81 enhances Stp1 and Stp2 dependent transcription necessitating negative modulation by inner nuclear membrane protein Asi1 in Saccharomyces cerevisiae

The yeast transcription factors Stp1 and Stp2 are synthesized as latent cytoplasmic precursors. In response to extracellular amino acids, the plasma membrane SPS-sensor endoproteolytically excises the N-terminal domains that mediate cytoplasmic retention, enabling the processed forms to efficiently enter the nucleus and induce gene expression. Cytoplasmic retention is not absolute, low levels of full-length Stp1 and Stp2 "leak" into the nucleus, and the concerted action of inner nuclear membrane proteins Asi1, Asi2 and Asi3 restricts their promoter access. In cells lacking Asi function, the precursor forms bind promoters and constitutively induce gene expression. To understand the requirement of Asi-dependent repression, spontaneous mutations in Required for Latent Stp1/2-mediated transcription (RLS) genes that abolish the constitutive expression of SPS-sensor regulated genes in an asi1 strain were selected. A single gene, allelic with DAL81, was identified. We show that Dal81 indiscriminately amplifies the transactivation potential of both full-length and processed Stp1 and Stp2 by facilitating promoter binding. In dal81 mutants, the repressing activity of the Asi proteins is dispensable, demonstrating that without amplification, the levels of full-length Stp1 and Stp2 that escape cytoplasmic retention are insufficient to activate transcription. Conversely, the high levels of processed Stp1 and Stp2 that accumulate in the nucleus of induced cells activate transcription in the absence of Dal81.

Key Words: Saccharomyces cerevisiae, amino acid permeases, gene expression, inner nuclear membrane, latent transcription factor

Online ISS 1091-6490
Copyright 2008 by the Genetics Society of America
phone: 412-268-1812   fax: 412-268-1813   email: genetics-gsa{at}andrew.cmu.edu