微生物生理学课件.pptVIP

* * Catabolite Repression Enzyme Induction is still considered a form of negative control because the effect of the regulatory molecule (the active repressor) is to decrease or downregulate transcription.? Catabolite repression is a type of positive control of transcription, since a regulatory protein affects an increase (upregulation) in the rate of transcription of an operon. The process was discovered in E. coli and was originally referred to as the glucose effect because it was found that glucose repressed the synthesis of certain inducible enzymes, even though the inducer of the pathway was present in the environment. The discovery was made during study of the regulation of lac operon in E. coli. Since glucose is degraded by constitutive enzymes and lactose is initially degraded by inducible enzymes, what would happen if the bacterium was grown in limiting amounts of glucose and lactose? A plot of the bacterial growth rate resulted in a diauxic growth curve which showed two distinct phases of active growth (Figure 9). During the first phase of exponential growth, the bacteria utilize glucose as a source of energy until all the glucose is exhausted. Then, after a secondary lag phase, the lactose is utilized during a second stage of exponential growth. Figure 9. The Diauxic Growth Curve of E. coli grown in limiting concentrations of a mixture of glucose and lactose During the period of glucose utilization, lactose is not utilized because the cells are unable to transport and cleave the disaccharide lactose. Glucose is always metabolized first in preference to other sugars. Only after glucose is completely utilized is lactose degraded. The lactose operon is repressed even though lactose (the inducer) is present. The ecological rationale is that glucose is a better source of energy than lactose since its utilization requires two less enzymes. Only after glucose is exhausted are the enzymes for lactose utilization synthesized. The secondary lag during diauxic