Regulation of transcription

The strength of a promoter and alternative sigma factors represent two ways in which the cell can adjust the rate of mRNA transcription from a particular promoter and therefore the amount of individual proteins in the cell. There are some genes which seem to be switched on most of the time particularly those genes involved in ‘housekeeping’ functions of the cell such as central metabolic pathways and those which account for gene products like as the ribosomal components. However, we have come to recognize that all genes are regulated in some way at some stage during the cell cycle and the bacterial cell has a variety of means of altering the flow of information from the genome to the proteome.

The two most general forms of regulation cited in the study of bacteria are derepression where a protein bound to a promoter stopping transcription is removed and the gene is switched on and attenuation where the presence or absence of a substrate necessary for the function of the gene product governs the transcription of the gene itself. The former is exemplified through the E. coli lac operon the latter through the trp operon from the same organism. It should be noted that activation where the presence of a protein is used to switch a gene on is used much less frequently in Bacteria than derepression. The contrary is true in the Archaea and in eukaryotes where transcription factors are the main regulators in activating gene expression.