Regulation of Prokaryotic Gene Expression: Messenger RNA transcription from bacterial operons

In prokaryotes such as the bacterium Escherichia coli (E. coli), regulation of gene expression occurs primarily at the level of transcription and, in general, is mediated by the binding of trans-acting proteins to cis-acting regulatory elements on their single DNA molecule (chromosome). [Note: Regulating the first step in the expression of a gene is an efficient approach, insofar as energy is not wasted making unneeded gene products.] Transcriptional control in prokaryotes can involve the initiation or premature termination of transcription.
Messenger RNA transcription from bacterial operons
In bacteria, the structural genes that encode proteins involved in a particular metabolic pathway are often found sequentially grouped on the chromosome along with the cis-acting elements that regulate the transcription of these genes. The transcription product is a single polycistronic messenger RNA ([mRNA]). The genes are, thus, coordinately regulated (that is, turned on or off as a unit). This entire package is referred to as an operon.