Gene expression (prokaryotes): additional lac operon practice

The regulation of gene expression in Escherichia coli occurs largely at the level of transcription via operons. An operon is a segment of DNA containing a cluster of genes, which are all under the control of a single promoter and the same regulatory elements. The genes in an operon are transcribed together into a single mRNA strand and therefore undergo the same transcriptional regulation.

The lac operon, in particular, regulates the production of genes used in the metabolism of lactose. The maximal transcription of the lac operon occurs when glucose, E. coli's preferred energy source, is absent and lactose is present. In addition to the promoter, the lac operon also has a segment of DNA called an operator, where regulatory proteins bind to transcriptionally regulate the operon. The operon is regulated by the presence or absence of lactose.

The main regulatory gene that acts on the lac operon is a repressor, which is constitutively produced. In the absence of lactose, the repressor binds to the lac operon operator, where it blocks RNA polymerase from transcribing the lac operon. The proteins encoded by the lac operon (not necessarily in this order) are permease, beta-galactosidase, and transacetylase.

In the presence of lactose, a lactose metabolite, allolactose, binds to the repressor protein. This causes the repressor to change conformation, preventing its binding to the lac operon operator. RNA polymerase then binds to the lac operon promoter and initiates the synthesis of the mRNA for the three structural genes. The genes in this operon are always transcribed in the same order.

A researcher designs an experiment to determine the order of genes in the lac operon. They selectively blocked RNA polymerase before it transcribed each gene in the operon and then measured the protein levels of each of the structural genes of the lac operon. The results are show in Table 1.

Table 1 Effects of blocking RNA polymerase prior to transcription of sequential genes