13 Signalling and Regulatory Circuits in Yeast [biochemie.web.med.******@2Fmuenchen.de@2FYeast_Biology@2F13_regulation.pdf].pdf

13 Signalling and Regulatory Circuits in Yeast
The coordination plex cellular processes - from growth and differentiation to responses of environmental changes - are usually
dpends on the careful regulation of gene expression. Broadly speaking, most changes in gene expression are brought about by either
the activation or the repression of gene transcription. Transcriptional activation is relatively well studied and involves the interaction of
sequence-specific DNA-binding proteins with recognition sites on DNA, recruitment of RNA polymerase and its associated factors, and
the initiation of transcription. Transcriptional repression is more difficult to define because, in a sense, a gene present in a cell type that
lacks its necessary activators can be said to be repressed. One aspect of gene repression is 'active repression': the specific repression
of a gene or a set of genes, even when the necessary activators are present in a cell. Eukaryotes have several ways of bringing about
the active repression of selected genes, including modulation of the local acetylation state of histones, modulation of the chromatin
structures, interference with activators or with the transcriptional machinery directly.
A further aspect of gene regulation is that transcriptional activation as well as transcriptional repression are dependent on the
environmental conditions of a cell. Growth factors, nutrients, oxygen supply, osmotic parameters, pH, or temperature will control gene
expression, and this type of control is mediated by signal cascades which transform external stimuli into cellular responses. Likewise,
intracellular changes such as DNA damage or aberrations in metabolic capacities are sensed and invoke reactions of the transcriptional
processes.
MAP Kinase Pathways in Yeast
Yeasts posseses rapidly responding, plex signalling pathways that allow cells to quickly adapt to a changing environment, a
critical attribute for a nonmotile species. Wi