Transcriptional Regulation In Animals

2.0. Review of the literature

Transcriptional regulation is a tightly controlled process which involves the genetic and epigenetic regulation of genes at both transcriptional and post transcriptional levels in response to various environmental stimuli. It encompasses many of the processes which lead to either activation or repression of genes. Most of the mechanisms of transcriptional regulation are common in plants and animals but plants being sessile have a greater complexity of regulation that requires them to adjust to their surrounding environment. There are several unique pathways (phyto-hormone signaling, light-response and abiotic stress pathways) that are only found in plants. Alterations to the chromatin structure such as DNA methylation

Transcription factors play important and diverse roles in gene expression, including chromatin remodeling and recruitment/stabilization of the Pol II transcription-initiation complex. Often, specificity in a given process or in a given tissue or cell type is brought about by recruitment of process/stage/tissue/cell specific factors. Specificity of gene expression also depends on cis elements present in the promoter and enhancer regions and their interaction with specific transcription factors. Transcription factors can be divided into a number of functional classes (Singh, 1998). They may function as activators or repressors of transcription. They bind to specific DNA sequences found only in certain promoters and regulate gene expression either through activation of transcription or repression of transcription of the gene downstream. The function of several transcription factors is aided by certain proteins known as co-activators and co-repressors. Co-activators or co-repressors proteins are regulators that do not bind DNA on their own but interact with other activators or repressors via protein-protein interactions. A third class of regulators includes the general transcription factors (GTFs), which are important components of the RNA Pol II transcription complex. A fourth class

The basic unit of chromatin is the nucleosome core, in which 146 bp of DNA is wrapped around a histone octamer. Each histone octamer consists of a dimer of four histone proteins H2A, H2B, H3 and H4. These are basic proteins, responsible for the interaction of DNA with the core octamer. Covalent modification of the tails of histones and non-histone proteins regulate the packaging and unpackaging of DNA and are thus responsible for gene activation & repression by chromatin structure modifications. Histone tails are the site for various post translational modifications including acetylation, phosphorylation, methylation and ubiquitylation. Of these, histone acetylation correlates with transcriptional activation and histone deactylation with transcriptional repression. The acetylation of histones occurs on Lys residues in the amino-terminal tails of histones leading to neutralization of the positive charge of the histone tails thus reducing their affinity for DNA. This in turn leads to unfolding of the nucleosome making the DNA more accessible for transcription. In contrast, deacetylation of histones removes the negative charge of the acetyl group, thus restoring the positive charge on histones and increasing their affinity for DNA thereby repressing transcription. The enzymes that are responsible for acetylation and deacetylation are known as histone acetyl