A paradoxical increase of a metabolite upon increased expression of its catabolic enzyme: the case of diadenosine tetraphosphate (Ap4A) and Ap4A phosphorylase I in Saccharomyces cerevisiae.

The APA1 gene in Saccharomyces cerevisiae encodes Ap4A phosphorylase I, the catabolic enzyme for diadenosine 5',5"'-P1,P4-tetraphosphate (Ap4A). APA1 has been inserted into a multicopy plasmid and into a centromeric plasmid with a GAL1 promoter. Enhanced expression of APA1 via the plasmids resulted in 10- and 90-fold increases in Ap4A phosphorylase activity, respectively, as assayed in vitro. However, the intracellular concentration of Ap4A exhibited increases of 2- and 15-fold, respectively, from the two different plasmids. Intracellular Ap4A increased 3- to 20-fold during growth on galactose of a transformant with APA1 under the control of the GAL1 promoter. Intracellular adenosine 5'-P1-tetraphospho-P4-5"'-guanosine (Ap4G) and diguanosine 5',5"'-P1,P4-tetraphosphate (Gp4G) also increased in the transformant under these conditions. The chromosomal locus of APA1 has been disrupted in a haploid strain. The Ap4A phosphorylase activity decreased by 80% and the intracellular Ap4A concentration increased by a factor of five in the null mutant. These results with the null mutant agree with previous results reported by Plateau et al. (P. Plateau, M. Fromant, J.-M. Schmitter, J.-M. Buhler, and S. Blancquet, J. Bacteriol. 171:6437-6445, 1989). The paradoxical increase in Ap4A upon enhanced expression of APA1 indicates that the metabolic consequences of altered gene expression may be more complex than indicated solely by assay of enzymatic activity of the gene product.