Molecular cloning and characterization of the STA2 glucoamylase gene of Saccharomyces diastaticus

Summary

The Saccharomyces diastaticus structural gene STA2, encoding an extracellular glucoamylase (1,4-α-d-glucan glycohydrolase, EC 3.2.1.3.), has been cloned by complementation of a sta ^o strain. A genomic library was initially constructed from a STA2 yeast strain in the yeast Escherichia coli shuttle cosmid vector pYCl. The Sta⁺ complementing function was further delimited to an 8.3 kb BglII fragment whose restriction map was found to be similar to related genomic regions of STA1 and STA3. Fusions of several DNA fragments derived from the 8.3 kb BglII fragment with a truncated E. coli β-galactosidase gene resulted in two overlapping fragments that could direct the production of large fusion proteins in E. coli. These fusion proteins were immunoprecipitable by anti-glucoamylase II antibodies, confirming that the Sta⁺ complementing fusion was due to the expression of a gene that coded for a yeast glucoamylase. Measurements of the STA1, STA2 and STA3 RNA transcripts by RNA-DNA hybridization using an internal fragment of the cloned STA2 gene as the probe indicated that a common transcript of 2.5 kb is produced by each of the STA genes. Integrative disruption of the STA2 gene through homologous recombination was achieved by transforming a STA2 yeast strain to Sta⁻ using an in vitro constructed donor DNA fragment that has the URA3 gene inserted within the coding region of the cloned glucoamylase gene. This was confirmed by tetrad analysis of crosses between strains carrying a disrupted STA2 and a functional STA2. Southern blot analysis using BamHI digested genomic DNA from 15 tetrads demonstrated consistent co-segregation and Mendelian inheritance of the Sta⁻ phenotype with STA2::URA3. These data further confirm that the cloned DNA that showed Sta⁺ complementing activity carries a functional STA2 gene that encodes the yeast extracellular glucoamylase II.