“The operon is a fundamental mechanism for regulation of gene expression. An operon is a sequence of DNA which generally consists of a set of structural genes, a promoter, a regulator, and an operator. The promoter is an RNA polymerase binding site which allows for transcription of the structural genes. The operator serves as a binding site for a repressor molecule. The regulator gene codes for the repressor protein, which controls the expression of the structural genes. The structural genes are frequently transcribed together as a single mRNA molecule from which multiple proteins are made.

The lac operon allows for the metabolism of lactose by the E. coli bacterium. See Figure 1. The structural genes in the lac operon include lacZ and lacY, which produce β-galactosidase and lactose permease, respectively. Lactose permease transports lactose into the cell, where β-galactosidase cleaves it into glucose and galactose. The genes of the lac operon are only expressed when lactose is present and the operator is not bound by a repressor. Transcription from the promoter is upregulated in the presence of cyclic AMP (cAMP), which forms a complex with the cAMP receptor protein (CRP) and binds to the CRP binding site. The lacI gene is constitutively expressing repressor proteins which bind the operator in the absence of lactose. Table 1 describes the transcriptional activity of the lac operon.

Figure 1. The lac operon. CRP = the Cyclic AMP Receptor Protein binding site; P = promoter; O = operator.

 

Glucose	Lactose	CRP bound	Operator bound	lacZ	lacY
Y	N	N	Y	–	–
N	Y	Y	N	++	++
Y	Y	N	N	+	+

Table 1. Transcriptional activity of the lac operon. Y or N = Yes or No. CRP bound and Operator bound indicate whether or not the CRP or repressor proteins are bound to their respective binding sites. ++ = strong transcription. + = weak transcription. – = no transcription.”
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