Abstract
Studying the regulation of transcription of the gal operon that encodes the amphibolic pathway of d-galactose metabolism in Escherichia coli discerned a plethora of principles that operate in prokaryotic gene regulatory processes. In this chapter, we have reviewed some of the more recent findings in gal that continues to reveal unexpected but important mechanistic details. Since the operon is transcribed from two overlapping promoters, P1 and P2, regulated by common regulatory factors, each genetic or biochemical experiment allowed simultaneous discernment of two promoters. Recent studies range from genetic, biochemical through biophysical experiments providing explanations at physiological, mechanistic and single molecule levels. The salient observations highlighted here are: the axiom of determining transcription start points, discovery of a new promoter element different from the known ones that influences promoter strength, occurrence of an intrinsic DNA sequence element that overrides the transcription elongation pause created by a DNA-bound protein roadblock, first observation of a DNA loop and determination its trajectory, and piggybacking proteins and delivering to their DNA target.
Highlights
A closed DNA loop of 11 helical turns, which is in-flexible to torsional changes, disables the promoters either by resisting DNA unwinding needed for open complex formation or by impeding the processive DNA contacts by an RNA polymerase in flux during transcription initiation
The involvement of HU in DNA looping and the repression of the gal operon were investigated in vivo by monitoring the activity of -glucuronidase from gusA fused to the P2 promoter
The gal operon of E. coli plays an important role in cellular metabolism by encoding enzymes that catalyze conversion of D-galactose to energy sources as well as to anabolic substrates
Summary
The study of the galactose (gal) operon, which encodes enzymes for an amphibolic pathway of. D-galactose metabolism, first revealed a plethora of gene regulatory mechanisms by which bacterial genes are regulated: (i) beside substrate induction of a specific catabolic pathway or end-product repression of a specific biosynthetic pathway, accumulation or depletion of metabolic intermediates in the cell globally regulates the expression of a wide variety of genes to compensate for the accumulation or depletion [1]; (ii) use of more than one promoter to regulate an operon [2]; (iii) the mechanism of. We review more recent revelations that provide several new aspects of the multiple regulatory pathways by which gal promoters are regulated at the transcription level
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