REGULATION OF UDPG-PYROPHOSPHORYLASE ISOFORMS IN SACCHAROMYCES-CEREVISIAE AND THEIR ROLES IN TREHALOSE METABOLISM

UDPG-pyrophosphorylase (EC 2.7.7.9) from Saccharomyces cerevisiae was studied and the presence of isoforms investigated. Its activity was mo nitored during growth of cultures in rich media containing glucose, ga lactose, sucrose, maltose or glycerol as carbon sources. The results s uggest that UDPG-pyrophosphorylase is subject to both catabolite repre ssion and catabolite inactivation. The inactivation process seems to b e complex: in order to produce maximum inactivation, glucose and ammon ium sulfate must be added together. Addition of glucose or ammonium su lfate separately produced little effect upon enzyme activity. Adsorpti on to and elution from a DEAE-Sephacel column of a crude protein extra ct prepared from yeast cells collected in stationary phase from a gluc ose medium showed three activity peaks, which we denominated isoform I , II and III. Isoform I is constitutive, it was the only form present during exponential growth on glucose medium, and did not suffer any al teration after glucose exhaustion, heat shock or by growing cells on m altose. On the other hand, isoforms II and III were shown to be repres sed by glucose, and induced by heat shock. Furthermore, isoform II of UDPG-pyrophosphorylase was present together with isoform I when yeast cells were grown on maltose. The presence of a MAL4(C) allele rendered isoform II constitutive. Interestingly, a gal3 mutant strain had low UDPG-pyrophosphorylase activity and isoforms I and II were not express ed. These results are discussed in relation to trehalose metabolism.