Control of Gene Expression Bacteria

Control of Gene Expression
Bacteria have developed mechanisms to adapt quickly and efficiently to changes and triggers from the environment. This allows them to coordinate and regulate the expression of genes for multicomponent structures or the enzymes of one or more metabolic pathways. For example, temperature change could signify entry into the human host and indicate the need for a global change in metabolism and up-regulation of genes important for parasitism or virulence. Many bacterial genes are controlled at multiple levels and by multiple methods.
A coordinated change in the expression of many genes, as would be required for sporulation, occurs through use of a different sigma factor for the RNA polymerase. This would change the specificity of the RNA polymerase and allow mRNA synthesis for the necessary genes while ignoring unnecessary genes. Bacteria might produce more than six different sigma factors to provide global regulation in response to stress, shock, starvation, or to coordinate production of complicated structures such as flagella.
Coordination of a large number of processes on a global level can also be mediated by small molecular activators, such as cyclic adenosine monophosphate (cAMP). Increased cAMP levels indicate low glucose levels and the need to utilize alternative metabolic pathways. Similarly, in a process called quorum sensing, when a sufficient number of bacteria are present and producing a specific small molecule, virulence and other genes are turned on. The trigger for biofilm production by Pseudomonas spp. is triggered by a critical concentration of N-acyl homoserine lactone (AHL) produced when sufficient numbers of bacteria (a quorum) are present. Activation of toxin production and more virulent behavior by Staphylococcus aureus accompanies the increase in concentration of a cyclic peptide.
To coordinate the expression of a more limited group of genes, such as for a specific metabolic process, the genes for the necessary enzymes would be organized into an operon. The operon would be under the control of a promoter or repressor DNA sequence that can activate or turn off the expression of a gene or a group of genes to coordinate production of the necessary enzymes and allow the bacteria to react to changes in concentrations of nutrients. The genes for some virulence mechanisms are organized into a pathogenicity island under the control of a single promoter to allow their expression under appropriate (to the bacteria) conditions. The many components of the type III secretion devices of E. coli, Salmonella, or Yersinia are grouped together within a pathogenicity island.
Transcription can also be regulated by the translation process. Unlike eukaryotes, the absence of a nuclear membrane in prokaryotes allows the ribosome to bind to the mRNA as it is being transcribed from the DNA. The position and speed of ribosomal movement along the mRNA can affect the presence of loops in the mRNA and the ability of the polymerase to transcribe new mRNA. This allows control of gene expression at both the transcriptional and translational levels.
Initiation of transcription may be under positive or negative control. Genes under negative control are expressed unless they are switched off by a repressor protein. This repressor protein prevents gene expression by binding to a specific DNA sequence called the operator, blocking the RNA polymerase from initiating transcription at the promoter sequence. Inversely, genes whose expression is under positive control are not transcribed unless an active regulator protein, called an apoinducer, is present. The apoinducer binds to a specific DNA sequence and assists the RNA polymerase in the initiation steps by an unknown mechanism.
Operons can be inducible or repressible. Introduction of a substrate (inducer) into the growth medium may induce an operon to increase the expression of the enzymes necessary for its metabolism. An abundance of the end products (co-repressors) of a pathway may signal that a pathway should be shut down or repressed by reducing the synthesis of its enzymes.