Some gene products are required at all time and their genes are expressed at more or less constant level in all cells of a species or organism. The genes for the enzymes that catalyse the steps in a central metabolic pathway such as citric acid cycle fall in this category. These genes are called housekeeping genes and they are expressed at all time. This is called ‘constitutive gene expression’. The amount of gene products rises and falls in response to molecular signals.
Gene expression can be controlled at any of one of the stages, namely transcription, processing and translation. Transcription often is controlled at the stage of initiation. Transcription is not usually controlled at elongation, but may be controlled at the stage of termination. A prokaryotic cell could shut off the supply of unwanted protein at the transcription level by blocking the mRNA synthesis or transcriptional control mediated by repressor proteins or hydrolyse the mRNA once it is formed (post-transcriptional control). In bacteria, an mRNA is typically available for translation while it is being synthesized; this is called coupled transcription/translation. Translation in bacteria is regulated but passively. The gene activity is regulated by the specific interactions of the ‘trans-acting products’ (proteins) with sites on the DNA (‘cis-acting DNA sequences’).
The sequences that mark the beginning and end of the transcription unit—the promoter and the terminator are the examples of cis-acting sequences. A promoter serves to initiate transcription only of the genes that are present on the same stretch of DNA. Likewise, a terminator can terminate transcription only by an RNA polymerase that has traversed the preceding genes. That is both promoter and terminator are both cis-acting sequences that are recognized by the same trans-acting protein, i.e., RNA polymerase.
The bacterial genome is organized so that genes with a common function, such as the synthesis of enzymes involved in the biosynthetic or degradative pathway are often grouped together and are regulated together in units that are called as ‘operons’. The genes of the operon are transcribed into polycistronic mRNA that can be activated or repressed. Operons mainly comprise two main regions; namely, the ‘structural genes’ and the ‘regulatory genes’. A structural gene is simply any gene that codes for a protein or an RNA product. The structural genes include the genes of proteins that perform enormous functions. A regulatory gene codes for a protein that controls gene expression by binding to particular DNA sequences. Operons consist of several structural genes, a promoter, a terminator, a control site (operator) and a separate regulator gene (codes for protein that binds to operator).