Synthesis and Activation of Asparagine in Asparagine Auxotrophs of Saccharomyces cerevisiae

l ‐Asparagine synthesis in Saccharomyces cerevisiae is performed by a glutamine‐dependent asparagine synthetase of the type found in higher organisms. Auxotrophy for asparagine has been obtained in two classes of mutants. In class I, asparagine synthetase activity is cancelled. These mutants combinetwo mutations, asnA − and asnB − . Neither asnA − nor asnB − mutation alone. leads to total auxotrophy. Partial auxotrophy as well as a strong decrease in enzyme activity result from asnA − mutation. No change is detectable in cells with the asnB − mutation alone. This, and Jones' report [ J. Bacteriol. 134 , 200–207 (1978)] of auxotrophy resulting from the combination of two mutations, are strong supports for asparagine synthesis being an unusual biosynthetic operation. In class II, auxotrophy results from a single mutation which leads to a modification of the efficiency of the asparaginyl‐tRNA synthetase ( asnRS − mutation). This auxotrophy is cancelled if asparaginase I activity (the only one present in ∑ 1278b wild type) is cancelled by casnI − mutation. This latter mutation allows an increase in the asparagine pool which is able to compensate for the asparaginyl‐tRNA synthetase partial defect of the asnRS − mutant.