Aerobiosis Requirement of Diagnostic Color Differentiation for Respiration Deficiency in Yeast.

essentially unchanged. The dehydrase activity, however, declined until none was detectable. (This decline commenced approximately 1 hr after maximal growth was attained.) At this latter point, the cells were viable when placed in minimal medium, but there was a 2-hr "lag" period before growth commenced. It is tempting to interpret this decline as an expression of enzyme repression, but the present in vitro evidence on the dehydrase supports the contention that the decline is due not to repression but, rather, to the instability of the enzyme within the cell. The labile nature of this enzyme is a critical feature when studies on cell-free extracts are carried out. For example, dehydrase activity in extracts was unstable not only upon storage (60% loss of activity after overnight freezing) but also during maintenance of freshly prepared extracts in ice. Various attempts to stabilize the activity have been unsuccessful to date. Thus, to obtain valid results, it was necessary to carry out the dehydrase assays within 3 hr after preparation of the cell-free extracts. Further evidence of the instability of the enzyme includes the observation of Armstrong et al. that high levels of activity are unstable in the in vitro assay when various amino acids are present and the observation that activity is lost during dialysis (unpublished data). The dehydrase is the only enzyme of the common pathway that displays this marked degree of instability and that requires careful and rapid handling. The instability of the enzyme may account for the severe depression of the dehydrase activity that Armstrong et al. found to occur temporarily during the early period of growth of S. typhiV-