Saccharomyces Cerevisiae Null Mutants in Glucose Phosphorylation: Metabolism and Invertase Expression

A congenic series of Saccharomyces cerevisiae strains has been constructed which carry, in all combinations, null mutations in the three genes for glucose phosphorylation: HXK1, HXK2 and GLK1, coding hexokinase 1 (also called PI or A), hexokinase 2 (PII or B), and glucokinase, respectively: i.e., eight strains, all of which grow on glucose except for the triple mutant. All or several of the strains were characterized in their steady state batch growth with 0.2% or 2% glucose, in aerobic as well as respiration-inhibited conditions, with respect to growth rate, yield, and ethanol formation. Glucose flux values were generally similar for different strains and conditions, provided they contained either hexokinase 1 or hexokinase 2. And their aerobic growth, as known for wild type, was largely fermentative with ca. 1.5 mol ethanol made per mol glucose used. The strain lacking both hexokinases and containing glucokinase was an exception in having reduced flux, a result fitting with its maximal rate of glucose phosphorylation in vitro. Aerobic growth of even the latter strain was largely fermentative (ca. 1 mol ethanol per mol glucose). Invertase expression was determined for a variety of media. All strains with HXK2 showed repression in growth on glucose and the others did not. Derepression in the wild-type strain occurred at ca. 1 mM glucose. The metabolic data do not support- or disprove-a model with HXK2 having only a secondary role in catabolite repression related to more rapid metabolism.