A recombinant Saccharomyces cerevisiae industrial biotechnology (Barthel and Kula, 1993; Das C468/pGAC9 (ATCC 20690), which expresses Aspergillus and Schultz, 1987;Shi et al., 1993). awamori glucoamylase gene under the control of the In commercial applications of recombinant cells a yeast enolase I (ENO1) promoter and secretes glucoamymajor concern is the plasmid instability in plasmid-based lase into the extracellular medium, was used as a model systems. The instability relates to the tendency of the system to investigate the effect of cell immobilization on bioreactor culture performance. Free suspension cultures transformed cells to lose their engineered properties in stirred-tank and airlift bioreactors confirmed inherent due to the change or loss of plasmids. This problem genetic instability of the recombinant yeast. An immobiis especially important in recombinant yeasts because lized-cell-film airlift bioreactor was developed by emalmost all yeast plasmid vectors are hybrid plasmids, ploying cotton cloth sheets to immobilize the yeast cells so-called ''shuttle vectors,'' which are relatively unstaby attachment. Enhanced enzyme productivity and production stability in the immobilized-cell system were ob-ble. Two kinds of plasmid instability are responsible for served. Experimental data indicated that the immobilized change and/or loss of plasmids. Structural instability is cells maintained a higher proportion of plasmid-bearing often caused by the DNA's deletion, insertion, recombicells for longer periods under continuous operation. The nation, or other events, while segregational instability higher plasmid maintenance with immobilized cells is is caused by uneven partition of plasmids during cell possibly due to reduced specific growth rate and increased plasmid copy number. Double-selection pressure division (Primrose and Ehrlich, 1981; Ryu and Lee, was used to select and maintain the recombinant yeast.
1988). The genetic instability combined with the effects
The selected strain showed better production perforof environmental factors cause recombinant cells to lose mance than the original strain.