Transcription Regulatory Network

What is Transcription Regulatory Network?

Transcriptional regulation of gene expression in a cell is important to the survival and development of any organisms. For example, bacteria living in an environment of high lactose but no glucose (which is their “food source”) need to be able to alter the expression of certain genes to produce a protein that can break down lactose into glucose in order to survive and reproduce (see lac operon below). Similarly, cellular specialization of a multi-cellular organism requires a coordinated effort of turning genes “on” and “off” at specific times so that different cells can become specialized in different ways according to the proteins they express. These tasks of regulating gene transcriptions are achieved by collections of regulatory proteins and their interactions with specific sequences in the promoter regions of targeted genes. A “Transcriptional Regulatory Network” describes such regulatory proteins and interactions.

Transcriptional regulatory networks in viruses and prokaryotes:

Transcription regulatory proteins, often known as “activators” (activate gene transcriptions) or “repressors” (repress gene transcriptions), can be found to function in the relatively simpler genomes of viruses and prokaryokes. For example, the bacteriophage lambda alters its gene expression depending on whether it is entering the lytic cycle or the lysogenic cycle .

Although the phage lambda genome is only 48,502 base pairs long, the regulatory network that allows the phage to choose and follow one of the two cycles is quite complex already and still remains to be fully elucidated.

Figure: The phage lambda genome. (Source: Dodd, Ian B., Shearwin, Keith E., and Egan, J. Barry. Revisited gene regulation in bacteriophage lambda. 2005. Current Opinion in Genetics & Development. 15 (2): 145-152. )

Figure: The phage lambda regulatory network. (Source: Dodd, Ian B., Shearwin, Keith E., and Egan, J. Barry. Revisited gene regulation in bacteriophage lambda. 2005. Current Opinion in Genetics & Development. 15 (2): 145-152. )

For bacteria, the transcription regulatory network has been most extensively studied in E.coli. We have mentioned the need for the bacteria to metabolize lactose in the absence of glucose. In E.coli, the lac operon , a piece of DNA that contains a promoter region, an operator region and three genes, serves to regulate this process. The lac operon represents perhaps the simplest transcription regulatory mechanism; it involves a repressor protein, which is transcribed by the lacI gene and binds to the operator region, and the operator sequence itself. The presence of lactose serves to inactivate the repressor and thus allow for the transcriptions of the genes. Still, this transcription regulatory mechanism is only part of a bigger E.coli metabolic regulatory network, which may consist of as many as 149 genes, including genes that code for 16 regulatory proteins and 73 enzymes!