Transcription-translation coupling and polarity: a possible role of cyclic AMP

Gene regulation in continuous cultures: a unified theory for bacteria and yeasts

During batch growth on mixtures of two growth-limiting substrates, microbes consume the substrates either sequentially (diauxie) or simultaneously. The ubiquity of these growth patterns suggests that they may be driven by a universal mechanism common to all microbial species. Recently, we showed that a minimal model accounting only for enzyme induction and dilution, the two processes that occur in all microbes, explains the phenotypes observed in batch cultures of various wild-type and mutant/recombinant cells (Narang and Pilyugin in J. Theor. Biol. 244:326-348, 2007). Here, we examine the extension of the minimal model to continuous cultures. We show that: (1) Several enzymatic trends, attributed entirely to cross-regulatory mechanisms, such as catabolite repression and inducer exclusion, can be quantitatively explained by enzyme dilution. (2) The bifurcation diagram of the minimal model for continuous cultures, which classifies the substrate consumption pattern at any given dilution rate and feed concentrations, provides a precise explanation for the empirically observed correlations between the growth patterns in batch and continuous cultures. (3) Numerical simulations of the model are in excellent agreement with the data. The model captures the variation of the steady state substrate concentrations, cell densities, and enzyme levels during the single- and mixed-substrate growth of bacteria and yeasts at various dilution rates and feed concentrations. This variation is well approximated by simple analytical expressions that furnish deep physical insights. (4) Since the minimal model describes the behavior of the cells in the absence of cross-regulatory mechanisms, it provides a rigorous framework for quantifying the effect of these mechanisms. We illustrate this by analyzing several data sets from the literature.

Transcriptional control of polarity in Escherichia coli by cAMP

In Escherichia coli, 3'5'-adenosine cyclic monophosphate (cAMP) and its receptor protein (CAP) are known to be involved in the control of transcription initiation of catabolic operons. In previous papers we have shown that the cAMP-CAP complex is also involved as a modulator of polarity in polycistronic transcription units. Furthermore we showed that there exists a functional relationship between this complex and the transcription termination protein, Rho. In this work, we measured mRNA synthesis corresponding to the promoter proximal and distal parts of the lac and gal operons by DNA-RNA hybridization. We show that in these operons the main polarity effect is essentially transcriptional and the cAMP-CAP complex decreases polarity by interfering with premature transcription termination.

Cyclic AMP and its receptor protein are required for expression of transfer genes of conjugative plasmid F in Escherichia coli

A number of cya and crp mutants of Escherichia coli HfrH were analyzed for several Tra functions of the F plasmid. The mutants were observed to be deficient in conjugal donor ability, absorption of phages MS2 and Q beta and surface exclusion. These defects were suppressed in cya mutants grown with cAMP supplementation. A cAMP concentration of 3 X 10(-4) M produced maximal suppression of donor ability defect in a cya strain. cAMP did not suppress the Tra- phenotype of crp mutants. Latent periods of MS2 were shorter in cya and crp bacteria. Phage T7 development appeared similar in wild type, cya, and crp cells. It is concluded that tra genes of F plasmid are expressed only to a small extent in cya and crp mutants and that cAMP and its receptor protein are required for the normal expression of tra genes.

The cya locus of Escherichia coli K12: organization and gene products

The cya gene region of Escherichia coli K12 has been cloned into plasmid pBR322. Detailed analysis of the locus, using in vitro recombination techniques as well as specific labelling of gene products has given information on the organization and products of the region. The cya gene is preceded by two DNA segments which behave as promoters of equivalent strength; a divergent, very strong, promoter is also present in the same control region. The former pair of promoters direct transcription of the cya gene which is expressed as a 95,000 dalton polypeptide; the latter promoter controls synthesis of a 40,000 dalton polypeptide. The cya gene product can complement a cya delta mutation when fully expressed or when expressed as a protein truncated at its carboxy terminal end.

Multiple regulation of the activity of adenylate cyclase in Escherichia coli

We have studied the correlation between the activities of adenylate cyclase (ATP pyrophosphatelyase-(cyclizing); EC 4.6.1.1) and in vivo rates of synthesis and intracellular concentrations of adenosine 3',5' cyclic monophosphate (cAMP) under various growth conditions in wild-type Escherichia coli and in mutants lacking or overproducing the cAMP receptor protein (CAP). We showed that when wild-type bacteria are grown in the presence of a variety of carbon sources the intracellular concentrations of cAMP are inversely related to the adenylate cyclase activities determined in permeabilized cells, suggesting that the carbon source-dependent modulation of cAMP levels is not directly related to the regulation of adenylate cyclase activity. In mutants lacking functional CAP (crp) the in vivo rates of cAMP synthesis are several hundred-fold higher than in the wild-type parent without a parallel increase of adenylate cyclase activities. In a strain carrying multiple copies of the crp gene and overproducing CAP the activity of adenylate cyclase is severely inhibited, although the in vivo rate of cAMP synthesis is similar to the parental strain. We interpret these results as indicating that CAP controls mainly the activity rather than the synthesis of adenylate cyclase.

Isolation and characterization of an Escherichia coli mutant affected in the regulation of adenylate cyclase

A mutant, cyaR1, affecting regulation of adenylate cyclase expression or activity is described. It was obtained as a thermoresistant revertant of a strain harboring a thermosensitive transcription termination factor, rho (rho-15). This mutant failed to synthesize adenosine 3',5'-phosphate and exhibited a carbohydrate-negative phenotype. A secondary mutation at the crp locus (crpC) restored the ability of the mutant to synthesize adenosine 3',5'-phosphate, enabled the expression of catabolite-sensitive operons, and conferred on the strain an extreme sensitivity to catabolite repression. In addition, we showed that the crpC mutation restored the pleiotropic carbohydrate-positive phenotype even in a delta cya background. We interpret this to mean that the adenosine 3',5'-phosphate receptor protein regulates negatively either the activity or synthesis of adenylate cyclase and that the cyaR1 mutation is either in a regulatory protein or a regulatory site of adenylate cyclase.