Metabolic engineering of yeast: the perils of auxotrophic hosts

Auxotrophic mutants may have physiological alterations and sensitivities wh ich are not generally recognized. Such features are shown here by observati ons that final cell densities attained by several leucine-auxotrophic Sacch aromyces cerevisiae strains depend differently on the initial leucine conce ntration in the medium. Furthermore, complementing such auxotrophic strains with the plasmid-based LEU2 selection marker resulted in different final c ell densities than chromosomal expression of LEU2 in the otherwise isogenic , prototrophic strains. These results warn that auxotrophic host-related ph ysiological influences overlay any metabolic effect of a cloned gene expres sed in such a host, clearly complicating interpretation of the effect of th at gene's product in scientific or metabolic engineering research.