A multiplicity of potential carbon catabolite repression mechanisms in prokaryotic and eukaryotic microorganisms

The discovery of cyclic AMP (cAMP) and its receptor protein in Escherichia coli and the convincing demonstration that these molecules mediate catabolite repression of the synthesis of carbohydrate catabolic enzymes led to the widespread belief that the phenomenon of catabolite repression in bacteria was understood. It is now recognized that cAMP-independent catabolite repression mechanisms are operative in both prokaryotic and eukaryotic microorganisms. New evidence has led to the identification of a diversity of cAMP-independent regulatory mechanisms that may mediate catabolite repression in bacteria. These mechanisms utilize (i) novel transcription factors, (ii) starvation-induced RNA polymerase sigma factors, and (iii) three evolutionarily distinct protein phosphorylating enzyme systems. Although these mechanisms are not fully understood, it is suggested that they exert their effects at the transcriptional level and that phosphorylation and allosteric control by regulatory proteins are involved in these processes.