Escherichia coli adenylate cyclase homepage

Adenylate cyclase is the enzyme that catalyzes the transformation of adenosine triphosphate (ATP) into cyclic adenosine 3’, 5’-monophosphate (cAMP).

In Escherichia coli and related bacteria, cAMP controls gene expression [MMBR].  This control occurs via the Catabolite gene Activator Protein (CAP) [PNAS] also called Cyclic AMP Receptor Protein (CRP) [PNAS].  CRP exists as a homodimer.  Each subunit is composed of two domains connected by a small hinge, the N-terminal cAMP-binding domain and the C-terminal DNA-binding domain.  When bound to cAMP, CRP binds to specific sites upstream of promoters, causing transcriptional activation or repression [Annual Review of Biochemistry].

The mechanisms of CAP-dependent activation of transcription at class I and II promoters have been described by S. Busby and R. Ebright [Medline] and further characterized [Medline] [Biochemical Society Transactions] [FEMS Microbiology Letters].  The CRP-cAMP complex can also acts as a transcriptional regulator by competing for DNA binding or forming complexes with other transcriptional regulators, for example CytR [JBC Online] [Medline].

For structural information visit the "CAP" page maintained by the Biochemistry Biocomputing Group, University College London.  Upon binding of cAMP, the long helices at the dimer interface reposition leading to activation of the CRP-cAMP complex [JBC Online].  The mechanism by which CRP-cAMP activates transcription initiation is still under scrutiny [Cell Biochemistry and Function].

The CAP-cAMP complex is a positive transcriptional regulator of a number of catabolite operons [PNAS] and as such plays a role in catabolite repression [Medline].  Catabolite repression, aka carbon catabolite repression, is the mechanism by which glucose or another rapidly metabolized carbohydrate exerts a continued inhibition on synthesis of catabolic enzymes.  Catabolite repression has been extensively studied in enteric bacteria, particularly in the case of the lactose operon [JB].  The role of cAMP as an antagonist of catabolite repression [Medline] is well established [Medline].  Cyclic AMP-independent mutants of CAP have been isolated that relieve catabolite repression [Medline].

When catabolite repression occurs, a transcriptome analysis revealed that transcription of specific genes may be enhanced by CRP-cAMP [JB].  Groups of genes displaying a transcriptional response under condition of catabolite repression were further defined using transcriptome data [BMC Microbiology].

In addition to its role in regulating transcription of catabolite operons, the CRP-cAMP complex also regulates transcription of many other genes including genes involved in flagella formation [JB], biofilm formation [JB], nitrogen assimilation [Molecular Microbiology] [Nucleic Acids Research], ammonia assimilation [BMC Microbiology], energy metabolism [JB], iron uptake [EJB], membrane functions [JB], transport system [JB], porin synthesis [Molecular Microbiology], outer membrane passage [JB], bacterial conjugation [JB], osmoprotection [JB], stress response [MMBR], acid resistance [JB], stringent response [Cell], interspecies communication [JB] [JB], multidrug resistance [JB], and genes encoding enzymes of pharmaceutical importance; for example pga [JBC Online].  Furthermore some of the genes that are induced under starvation conditions are subject to transcriptional control by CRP-cAMP [Medline].  In uropathogenic E. coli, CRP regulates the formation of pili associated with pyelonephritis (Pap pili) [Molecular Microbiology].

Early reports have indicated that CRP-cAMP may regulate the transcription of genes involved in cell division.  However, it was later established that CRP-cAMP is not essential for cell division [JB].

Even though it was reported that CRP-cAMP regulates the synthesis of enzymes belonging to the tricarboxylic acid (TCA) cycle, the TCA cycle flux does not seem to be affected by the absence of the CRP-cAMP complex [JB].  The effect of CRP-cAMP on the TCA cycle flux was however observed under certain specific conditions [JB].

Finally, novel predicted binding sites have being identified in Escherichia coli that may be functional for CRP [PNAS].  Unknown CRP regulons have also been revealed by using a run-off transcription/microarray analysis (ROMA) [Nucleic Acids Research].  Interestingly, CRP can bind to thousands of different sites without affecting transcription initiation of genes located within these sites [PNAS].

Visit Regulon DB [Nucleic Acids Research] which contains the E. coli K12 "Regulon CRP" page or, for a listing of CRP-regulated genes with a color-coded score prediction Tractor DB [Nucleic Acids Research].

Escherichia coli cyaA strains lacking adenylate cyclase are viable indicating that cAMP is not essential for survival [JB].  However, because of the transcriptional regulation by CRP-cAMP, cyaA strains (or crp strains lacking CRP) present some unique features.  For example, they do not grow on a large variety of carbon sources including lactose, maltose, galactose, arabinose, glycerol... [Medline], they are not motile and thereby are incapable of chemotactic responses, they are resistant to antibiotics including fosfomycin [AAC], fosmidomycin [Bioscience, Biotechnology, and Biochemistry], nalidixic acid, streptomycin and mecillinam [JB], they are pH-sensitive for growth [JB], and they are better hosts for recombinant expression systems, especially in stationary phase [Medline].

Therefore, by controlling gene expression, cAMP is involved in many aspects of Escherichia coli physiology.

The transcription of the cyaA gene is negatively regulated by CRP-cAMP [JBC Online], as well as the transcription of the crp gene [Medline].  However, in both cases the negative transcriptional control by CRP-cAMP does not seem to cause major effects in vivo, at least under current experimental conditions [Medline].  In particular, it was shown by using gene fusion techniques that neither cAMP nor CRP-cAMP plays a major role in transcriptional or translational regulation of cyaA expression [JB].  In Salmonella typhimurium, transcription of cyaA is also repressed by CRP-cAMP [Genetics].

The transcription of crp is additionally regulated by the DNA-binding protein FIS, with both FIS and CRP-cAMP being required for repression of crp transcription [The EMBO Journal].  Incidentally, the study of FIS has lead to the concept of a transcription factor acting as a local "topological homeostat" [Frontiers in Bioscience].  Additionally in vitro studies have indicated that FIS may play a role in catabolite repression [Molecular Microbiology].

The translation of adenylate cyclase mRNA is ineffective [Medline] as to prevent excessive synthesis of adenylate cyclase.  This can be attributed to the fact that overproduction of cAMP is lethal to Escherichia coli possibly due to an accumulation of methylglyoxal [JB] [Microbiology].

Finally, intracellular cAMP concentrations vary with the carbon source used for bacterial growth.  Therefore, any gene whose transcription is regulated by CRP-cAMP may have various levels of expression depending on growth conditions, especially the carbon source available to the bacteria.  Correspondingly, the CRP-cAMP complex has been implicated in the establishment of foraging-like behavior when the carbon source is unfavorable [JBC Online] [Nucleic Acids Research].