Practice question

When culture medium supplies two carbon sources with differing energy yields, growth pattern shows two exponential phases separated by brief lag, termed diauxie. After consumption of preferred substrate usually glucose, cells pause to reprogram transcriptome before second substrate utilization. Molecular basis is carbon catabolite repression. Glucose transport via phosphotransferase system dephosphorylates EIIA-Glc, which blocks secondary sugar permeases via inducer exclusion and inhibits adenylate cyclase lowering cAMP, so catabolic operons like lac, ara, mal remain uninduced because CRP cAMP complex unable to bind DNA. When glucose exhausted, EIIA phosphorylated, exclusion relieved, cAMP rises, CRP activates alternative operons, specific repressors like LacI released by allolactose, enzymes synthesized enabling second growth phase. Process illustrates hierarchical sugar utilization ensuring energy efficient allocation of ribosomes, avoiding simultaneous synthesis of many hydrolases. In nature mixed substrates sequential use shapes competition, cross feeding, niche specialization in gut microbiota where Bacteroides species sequentially degrade glycans and influence community dynamics.