Oxygen-dependent upregulation of transcription of alginate genes algA, algC and algD in Pseudomonas aeruginosa

The mRNA levels of algA, algC and algD genes increased, coordinately, in cells of the highly mucoid Pseudomonas aeruginosa 8821M grown under increasing dissolved oxygen tensions (DOT) of up to 70% of air saturation. These genes encode the bifunctional protein with phosphomannose isomerase (PMI) and GDP-mannose pyrophosphorylase (GMP) activities (algA), the phosphomannomutase (PMI) (algC) and the GDP-mannose dehydrogenase (GMD) (algD). These four enzyme activities are necessary for the synthesis of GDP-mannuronic acid, which is the activated sugar precursor for alginate polymerization. For growth-limiting DOT - lower than 10% of air saturation - the increase in mRNA levels of algA, algC and algD with oxygen concentration was accompanied by a strong increase in the activity of the encoded enzymes and the consequent increase in alginate synthesis. However, and despite the upregulation of alginate gene transcription by DOT above 10% of air saturation, the activities of the encoded enzymes either maintained (GMP and GMD) or decreased (PMI and PMM) their levels at high oxygen tensions, leading to a slight decrease in alginate synthesis. This has previously been attributed to the oxidative inactivation of alginate enzymes, particularly of PMM and PMI activities.