Genetics of Ethanol-Producing Microorganisms

Ethanol inhibition of Saccharomyces and Candida enzymes

Ethanol inhibition of several hydrolases (sucrase, maltase, trehalase, melezitase and cellobiase) has been measured in both highly ethanol-tolerant Saccharomyces strains (R) and in Candida strains less tolerant to ethanol (S). Cells were either grown in the presence of ethanol and the activities of the enzymes measured without preincubation in this alcohol ("in situ" inhibition assay), or the culture was grown in the absence of ethanol and the activities of the enzymes were determined after preincubation and in the presence of this compound ("in vitro" inhibition assay). Ethanol inhibition (Ki values) of sucrase, maltase, trehalase, and melezitase was quite different for these different enzymes in the same strain (R or S), but similar for the same enzyme in different strains (R and S). The Ki values for cellobiase, which is absent from the R strain, were higher when induced than at the basal level and higher in in vitro assays than in in situ assays. This suggests that the inhibition observed in situ is mainly the result of an inhibition of other proteins related to cellobiase (i.e., those involved in its synthesis) but not a direct inactivation of the enzyme by ethanol. Accordingly, when hybrids between Saccharomyces (R) and Candida (S) strains were constructed by protoplast fusion, and cellobiase was measured in the parental Candida strain and some of the hybrids, there was an increase in the Ki values in the in situ assays from 2.25% ethanol in Candida to 5.5% in some of the hybrids.

A DEX gene conferring production of extracellular amyloglucosidase on yeast

A DEX gene from Saccharomyces diastaticus (strain BRG536 alpha DEX1) has been cloned in the hybrid vector pJDB207. The gene is included within a 3.6-kb fragment and confers production of extracellular amylo-alpha-1,4-glucosidase (AMG) and, thereby the ability to hydrolyse starch or dextrins on Dex- strains of Saccharomyces cerevisiae. The cloned gene hybridizes to three fragments produced by ClaI digestion of DNA from BRG536; one of these (11 kb) cosegregates in crosses with DEX1, while another (10 kb) is present in all Dex+ and Dex- strains examined. Accumulation of extracellular AMG by Dex+ transformants is up to five-fold that of BRG536, and escapes regulation by the CDX1 gene under conditions of excess glucose. The enzyme produced by Dex+ transformants resembles that of BRG536 with respect to Mr (approx. 150 X 10(3)) and effects of temperature and pH. The cloned DEX gene can be used as a selectable marker for introducing recombinant plasmids into wild-type strains of S. cerevisiae.