Regulation ofGlyoxylate Metabolism in Escherichia coli K-12

Therelative contributions ofthedicarboxylic acid andthetricarboxylic acid cycles totheoxidative catabolism ofglyoxylate inEscherichia coli K-12werededuced by analysis ofmutant strains that wereblocked intheformation ofglyoxylate carboligaseandofmalate synthase G (the "glycolate form"ofmalate synthase). Mutant strains unable toformmalate synthase G wereunimpaired intheir ability tooxidize glyoxylate. Hence, thedicarboxylic acid cycle doesnotappear toplayanessential role inthis process. Organisms blocked inthesynthesis ofglyoxylate carboligase didnotoxidize glyoxylate atadetectable rate, indicating that wild-type organisms convert glyoxylate toacetyl-coenzyme A andoxidize itviathetricarboxylic acid cycle. Theforegoing evidence indicates that malate synthase G plays ananaplerotic roleduring growth withglycolate oracetate asthecarbon source. Theinvivo activity ofmalate synthase G wasnotdetectable whentheintracellular concentrationofacetyl-coenzyme A waslow,suggesting thatthissubstrate ora closely related metabolite exerts asensitive positive control overtheenzyme. Thesynthesis ofmalate synthase G appears tobeinduced directly byglycolate whichmaybe formed byaconstitutive reduced nicotinamide adenine dinucleotide phosphatedependent glyoxylate reductase inglyoxylate- oracetate-grown cells. Glycolate orglyoxylate canserveasa sole carbon source forEscherichia coli K-12.When oneofthese compounds isthegrowth substrate, a