Genetic control of yeast mannan structure. Isolation and characterization of mannan mutants.

The yeast Saccharomyces cerevisiae X2180, the strain most commonly used for genetic studies on yeasts, was treated with the mutagen ethyl methanesulfonate, and mutants defective in the synthesis of cell wall mannan were isolated. The initial selection of mutants was made on the basis of their failure to agglutinate with antiserum that was specific for the mannotetraose side chain of the mannan, the immunodominant group of this yeast strain, which has the structure crMan(l-+3)otMan(l+Z)aMan(l-t2)Man. Nonagglutinating cells were assumed to have modified mannan structures, and this was confirmed by chemical analysis of the isolated mutant mannans. Two classes of mutants were obtained. One was apparently defective in the formation of the cr-1+3-mannosyltransferase (mnnl) which is presumed to be involved in the addition of the terminal or-l-+3-linked mannose unit to form the mannotetraose side chain. The other class was defective in one of two a-1-+2-mannosyltransferases that must be involved in adding the two or-1+2-linked mannose units, one of them (mnn.2) directly to the a-l-+6-linked mannose units in the backbone of the polysaccharide and the other (mnn3) to this first side chain mannose unit. No mutant was obtained that was defective in forming the backbone. Since such a mutation would yield a cell completely lacking mannan, it might be lethal. Complementation studies on diploid crosses of representatives of the different mutant classes gave results that were consistent with the interpretations based on chemical analysis of the mutant mannans. Thus, diploids obtained from crosses between mnnl X mnn2, mnn2 X mnn3, or mnnl X mnn3 mutants all made mannan of the X2180 chemotype. We conclude that all of the mutations involve structural genes for the various mannosyltransferases that are required for mannan synthesis and not regulatory genes that control expression of a particular mannan chemotype.