Question

What role does structure and function play in gene expression , specifically the initiation of transcription?

Answer 1

Genes for products that are required at all times, such as those for the enzymes of central metabolic pathways, are expressed at a more or less constant level in virtually every cell of a species or organism. Such genes are often referred to as housekeeping genes. Unvarying expression of a gene is called constitutive gene expression. For other gene products, cellular levels rise and fall in response to molecular signals; this is regulated gene expression. Gene products that increase in concentration under particular molecular circumstances are referred to as inducible; the process of increasing their expression is induction. The expression of many of the genes encoding DNA repair enzymes, for example, is induced by high levels of DNA damage. Conversely, gene products that decrease in concentration in response to a molecular signal are referred to as repressible, and the process is called repression. For example, in bacteria, ample supplies of tryptophan lead to repression of the genes for the enzymes that catalyze tryptophan biosynthesis. Transcription is mediated and regulated by protein DNA interactions, especially those involving the protein components of RNA polymerase. The nucleotide sequences of promoters vary considerably, affecting the binding affinity of RNA polymerases and thus the frequency of transcription initiation. SomeAlthough housekeeping genes are expressed constitutively, the cellular concentrations of the proteins   they encode vary widely. For these genes, the RNA polymerase promoter interaction strongly influences the rate of transcription initiation; differences in promoter sequence allow the cell to synthesize the appropriate level of each housekeeping gene productThe basal rate of transcription initiation at the promoters of nonhousekeeping genes is also determined by the promoter sequence, but expression of these genes is further modulated by regulatory proteins. Many of

these proteins work by enhancing or interfering with the interaction between RNA polymerase and the promoter. The sequences of eukaryotic promoters are more variable than their prokaryotic counterparts. The three eukaryotic RNA polymerases usually require an array of general transcription factors in order to bind to a promoter. Yet, as with prokaryotic gene expression, the basal level of transcription is determined by the effect of promoter sequences on the function of RNA polymerase and its associated transcription factors.