The bottom-fermenting lager yeast Saccharomyces pastorianus has been proposed to be allotetraploid, containing two S. cerevisiae (Sc)-type and two S. bayanus (Sb)-type chromosomes. This chromosomal constitution likely explains why recessive mutants of S. pastorianus have not previously been reported. Here we describe the construction of a ura3 deletion strain derived from the lager strain Weihenstephan34/70 by targeted transformation and subsequent loss of heterozygosity (LOH). Initially, deletion constructs of the Sc and Sb types of URA3 were constructed in laboratory yeast strains in which a TDH3p-hygro allele conferring hygromycin B resistance replaced ScURA3 and a KanMX cassette conferring G-418 resistance replaced SbURA3. The lager strain was then transformed with these constructs to yield a heterozygous URA3 disruptant (ScURA3⁺/Scura3Δ::TDH3p-hygro, SbURA3⁺/Sbura3Δ::KanMX), which was plated on 5-fluoroorotic acid (5-FOA) plates to generate the desired Ura⁻ homozygous disruptant (Scura3Δ::TDH3p-hygro/Scura3Δ::TDH3p-hygro Sbura3Δ::KanMX/Sbura3Δ::KanMX) through LOH. This ura3 deletion strain was then used to construct a bottom-fermenting yeast transformant overexpressing ATF1 that encodes an enzyme that produces acetate esters. The ATF1-overexpressing transformant produced significantly more acetate esters than the parent strain. The constructed ura3∆ lager strain will be a useful host for constructing strains of relevance to brewing.

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Journal Article

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Murakami N, Miyoshi S, Yokoyama R, Hoshida H, Akada R, Ogata T

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