Studies on the Evolution of Aromatic Beta-Glucoside Catabolic Systems under Different Stress Conditions in Escherichia coli

Abstract

The genetic systems involved in the utilization of aromatic -glucosides in Escherichia coli consist of the bgl, asc, and chb operons, along with the bglA locus encoding phospho- -glucosidase A. The bgl and asc operons are cryptic (silent) systems, as their expression is insufficient for sugar utilization in wild-type strains. Mutations that activate these operons confer a Bgl phenotype.

The persistence of cryptic genes without accumulating deleterious mutations, despite being silent, is an evolutionary puzzle. Evidence suggests that these genes may be expressed under specific physiological conditions, conferring fitness advantages. This study investigates the role of aromatic -glucoside catabolic systems in combating nutrient stress and microaerobic growth conditions.

Chapter 2 - Evolution Under Nutrient Stress The bgl operon, primarily involved in arbutin and salicin utilization, also enables esculin utilization.

In the absence of bglB, activation of the silent asc operon allows esculin utilization.

The bglA gene, specific for arbutin, can evolve via successive mutations to hydrolyze esculin and salicin sequentially.

Mutants retain arbutin phenotype, showing enhanced enzyme promiscuity.

Esc mutant: carries a four-base insertion in the bglA promoter, enhancing transcription and mRNA stability.

Sal mutant: carries a T583G transversion in bglA, resulting in C195G substitution near the active site. Docking studies show altered substrate positioning, enabling salicin binding.

Conclusion: Exposure to novel substrates under nutrient deprivation drives evolution of new metabolic capabilities via sequential modifications of pre-existing systems.

Chapter 3 - Role Under Microaerobic Conditions Expression of the bgl operon is enhanced 22-fold under anaerobic conditions.

Evidence shows physiological relevance under low oxygen, mediated by the ArcAB two-component system.

lacZ reporter assays: transcription from the bgl promoter is enhanced under microaerobic/static conditions in the presence of arcA.

ArcA’s effect is absent in hns-null strains, indicating ArcA antagonizes H-NS repression.

Competition experiments: activated bgl alleles confer growth advantage under low oxygen.

Wild-type arcA enhances growth advantage under static conditions but not aerobic conditions.

Absence of bgl reduces the GASP phenotype conferred by arcA, suggesting bgl is a downstream target.

Different downstream genes are regulated under static vs aerobic conditions, implying distinct mechanisms.