Two recombinant strains of Saccharomyces cerevisiae expressing the BGL1 (beta-glucosidase) gene originating from Saccharonlycopsis fibuligera were characterized with respect to gene expression and growth on cellobiose under air and nitrogen gas phases. The laboratory strain Y294[ySF1], with beta-glucosidase expression from a multi-copy plasmid, grew at comparable rates on cellobiose and glucose under both air and nitrogen gas phases. By contrast, strain N96[ySF1] grew at a much slower rate on cellobiose than on glucose under both gas phases. For the various strain and substrate combinations tested, cell-specific enzyme activity was significantly higher under a nitrogen gas phase as compared to an air gas phase. The ability of these strains to grow on cellobiose, a non-native substrate, was evaluated in terms of a dimensionless 'sufficiency' parameter, S, consisting of the ratio of the maximum cell-specific rate of glucose production from cellobiose to the maximum cell-specific rate of glucose consumption. At sufficiency values substantially less than one, specific growth rates were found to be limited by heterologous enzyme expression, whereas for values of sufficiency near and greater to one, specific growth rates on cellobiose approached their values on glucose. The concept of sufficiency appears to have general utility for work aimed at growth enablement on non-native substrates by virtue of heterologous enzyme expression. (c) 2005 Elsevier Inc. All rights reserved.