FBA1 encodes a cytoplasmic enzyme that catalyzes the conversion of fructose 1,6 bisphosphate into two 3-carbon products: glyceraldehyde-3-phosphate and dihydroxyacetone phosphate (2,1, 3). Transcription of FBA1 is not regulated by glucose and high levels of expression have been observed during growth on a nonfermentable carbon source (5). The promoter region of FBA1 has been shown to contain a single upstream activation site (UAS) located between positions -550 and ?440 upstream of the aldolase open reading frame. This upstream positive regulatory element has also been shown to contain sequences known to constitute the binding sites for the transcription factors Rap1p and Abf1p and two TTCC motifs (5).

Fba1p also catalyzes the reverse reaction, the synthesis of fructose 1,6 bisphosphate from glyceraldehyde-3-phosphate and dihydroxyacetone phosphate 2. This reaction is important for growth on non-sugar carbon sources like ethanol, glycerol, or peptone, when the gluconeogenesis pathway is used to synthesize glucose.

The reactions of gluconeogenesis, shown here, mediate conversion of pyruvate to glucose, which is the opposite of glycolysis, the formation of pyruvate from glucose. While these two pathways have several reactions in common, they are not the exact reverse of each other. As the glycolytic enzymes phosphofructokinase (Pfk1p, Pfk2p) and pyruvate kinase (Cdc19p) only function in the forward direction, the gluconeogenesis pathway replaces those steps with the enzymes pyruvate carboxylase (Pyc1p, Pyc2p) and phosphoenolpyruvate carboxykinase (Pck1p) -generating oxaloacetate as an intermediate from pyruvate to phosphoenolpyruvate- and also the enzyme fructose-1,6-bisphosphatase (Fbp1p) (reviewed in 6). Overall, the gluconeogenic reactions convert two molecules of pyruvate to a molecule of glucose, with the expenditure of six high-energy phosphate bonds, four from ATP and two from GTP.

Last updated: 2005-06-17