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.