Galactose Operon

The galactose operon (Figure 26.38) controls the utilization of galactose, one of the products of lactose cleavage by lac operon enzymes. The gal operon is regulated negatively by a repressor in a manner comparable to lac regulation, except that the repressor gene (galR) is unlinked to the structural genes.

Overlapping promoters - The gal operon contains two overlapping promoters (S1 and S2, Figure 26.38), leading to transcripts that are initiated just five nucleotides apart.

> 1. Transcription from the S1 promoter depends on the presence of the cAMP - CRP complex (see here).

> 2. The S2 promoter, is used when glucose is present.

The details of this dual regulation are not clear, but it is significant that galactose has a biosynthetic fate in addition to its role as an energy substrate. UDP-galactose is used in synthesis of cell wall lipopolysaccharide, so the second promoter may exist to ensure that UDP-galactose is available even when the cell is using glucose as its prime energy source.

See also: cAMP receptor protein (CRP), Lac Repressor, trp Operon Regulation, Arabinose Operon