Regulación Transcripcional de la respuesta al estrés en la levadura Saccharomyces cerevisiae. Papel de los factores transcripcionales Msn2p y Msn4p.

Abstract

The Saccharomyces cerevisiae genes MSN2 and MSN4 encode homologous and functionally redundant Cys2Hys2 zinc finger proteins. A disruption of both MSN2 and MSN4 genes results in a higher sensitivity of yeast cells to different stresses, including carbon source starvation, heat shock and severe osmotic and oxidative stresses. In this work we show that Msn2p and Msn4p are required for the activation of several yeast genes such as CTT1, HSP104, DDR2 and HSP12, whose induction is mediated through stress-response elements (STREs). Msn2p and Msn4p are important factors for the stress-induced activation of STRE dependent promoters and bind specifically to STRE-containing oligonucleotides. Moreover, by in vivo footprinting we demonstrate that Msn2p and Msn4p bind in vivo to the STREs present in several stress gene promoters. We propose that Msn2p and Msn4p are the previously unknown STRE binding factors, or STRFs. In the second chapter of this work we analyse the causes of an experimentally observed fact: expression of invertase in the yeast Saccharomyces cerevisiae is greatly delayed when derepression occurs in a medium that lacks a usable carbon source. The delay is not a consequence of defects in the transcription of the SUC2 gene but is due to the impossibility of translating the normal levels of mRNA generating under derepressing conditions. In the absence of glucose a complete inhibition of translation occurs, and this causes the lag time before the new carbon source can be used, even if the mRNAs of glucose-repressible genes are transcripted early.