Carbon sources promoters, switching

It seems that to alleviate this problem, promotors for catabolic operons in E. coli form a hierarchy with respect to their affinity for RNA polymerase those for rapidly-metabolized carbon sources being the strongest and those for poor carbon sources the weakest. Moreover, the weaker the promoter, the higher the requirement for cyclic AMP and its receptor protein GRP for transcription to be initiated. Thus, powerful promoters are recognized by RNA polymerase in the virtual absence of cAMP/CRP and the operons transcribed, if induced. Promoters for poorer carbon sources which require cAMP/CRP for transcription can then be progressively switched on as the growth rate slows and the intracellular cAMP concentration rises until, the poorest carbon sources, which sustains the slowest growth rate (and hence... 

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