Download Key concepts_Regulation of transcription in Bacteria

Key concepts Regulation of Transcription in Bacteria
Within a bacterial cell, the expression of some genes is regulated to respond to environmental changes.
Other constitutive or housekeeping genes are transcribed uniformly to maintain cellular functions.
Regulation may occur in many ways: the most direct is through differences in promoter strength or the
use of alternative σ factors.
A general type of regulation in bacteria is the stringent response. Major changes in the expression of
many genes are induced when general starvation for critical nutrients occurs.
A more precise method of controlling transcription is the activation or repression of specific operons,
groups of genes with related functions and a common promoter in response to specific environmental
changes. This is frequently effected by the presence of certain transcription factors that can bind to the
DNA, usually in the promoter region.
Examples of control by transcription factors include the lactose operon, which has both repressor and
activator factors; the arabinose operon, which has one factor that can act as either activator or
repressor through a mechanism that involves DNA looping; and the tryptophan operon, which also
involves the linkage of translation and transcription that occurs in bacteria.
Transcription factors are usually allosteric proteins, in which one or another con-formational state with
specific DNA-binding capabilities is favored by the absence or presence of a small effector molecule.
Bacterial transcription can also be regulated by the control of DNA supercoiling, on either a global or
local scale.
DNA methylation can also affect the transcription of particular genes.
Regulation of transcription in bacteria is hierarchical; there are several distinguishable levels of control.
The most specific control occurs at the operon level. However, sets of operons with related functions,
called regulons, may have common control elements, and several functionally related regulons may be
grouped into a few modulons.