In lager beers, the content of DMS (dimethylsulfide) regularly exceeds the taste threshold level, giving rise to an unwanted sensation of "cooked sweet-corn". DMS found in beer may be derived through thermal degradation of S-methyl-methionine during kilning and wort preparation, but may also be formed in the brewing yeast by DMSO (dimethylsulfoxide) reductase. Yeast DMSO reductase was suggested to be identical to PMSR (peptide methionine sulfoxide reductase), and in a previous study we showed that inactivation of the PMSR-encoding MXR1 gene in Saccharomyces cerevisiae abolished formation of DMS from DMSO. We set out to construct Saccharomyces carlsbergensis brewing yeast strains without MXR1 activity, and to study the effects of the mutation on formation of DMS from wort DMSO, as well as the general fermentation properties of such yeasts. Two versions of the MXR1 gene, MXR1-CE and MXR1-CA, are present in S. carlsbergensis lager brewing yeast. The non-S. cerevisiae-like MXR1-CA gene was isolated, and inactive deletion alleles of both gene versions were constructed and substituted for their functional counterparts in strains of S. carlsbergensis. Such mxr1-delta mutants were not able to form DMS from DMSO at all. In pilot plant fermentation trials, the mutant yeasts were found to behave well, and the resulting beer to contain 50 to 80% less DMS than beer produced with a comparable reference brewing yeast, confirming the role of PMSR in formation of DMS from DMSO. Over-expression of the MXR1 gene from a gene promoter 16 times stronger than the native promoter resulted in a final beer DMS content increased by a factor of 8, further emphasising the importance of MXR1 gene activity during beer fermentation. In summary, we were able to demonstrate the crucial role of MXR1 gene activity for the DMS content in beer and to construct a functional lager brewing yeast without potential for DMS formation. The acquired knowledge may help to define more precisely the wort DMS potential, and the constructed strains of brewer's yeast without MXR1 gene activity could potentially be used for beer production from wort with a high DMSO content.