| Summary: | Diverse and distantly-related bacteria can develop competence for natural
transformation. Competent cells can bind free extracellular DNA, transport it into
the cytoplasm, and sometimes recombine it into the chromosome. Competence has
a long evolutionary history and is therefore expected to significantly benefit the cell.
In an attempt to elucidate the function (benefit) of natural competence, I have
carried out genetic studies of the regulation of competence development in
Haemophilus influenzae.
Competence in this organism is dependent on an increase in intracellular
concentrations of cAMP complexed with its receptor, CRP. In related bacteria, cAMP
synthesis by adenylate cyclase is regulated in response to carbon source availability
by the phosphoenolpyruvate:glycose phosphotransferase system (PTS). This
enzyme complex detects availability of preferred sugars, and transports them into
the cell. In the absence of preferred sugars, the PTS activates adenylate cyclase.
I demonstrated the existence of a simple fructose-specific PTS in H. influenzae by
cloning the pts and fru operons. I disrupted genes encoding PTS components,
constructed mutant strains, and assessed the effect of these mutations on
competence and other cAMP-dependent phenotypes. Strains lacking or unable to
activate the putative adenylate cyclase-regulating component of this PTS (EIIA[sup Glc])
showed a 150-fold reduction in competence under standard competence-inducing
conditions, unless exogenous cAMP was added. Moreover, these PTS-deficient
strains could not catabolize cAMP-dependent sugars, and showed reduced (3-
galactosidase expression from a cAMP-dependent /acZ-based reporter construct,
implying that the H. influenzae regulates adenylate cyclase activity and competence.
Competence was also found to be optimized by a cAMP-phosphodiesterase and
reduced by the presence of physiological concentrations of free nucleotides. Putative
regulatory sites in the promoters of competence genes were shown to be
indistinguishable from cAMP-CRP binding sites, suggesting that the cAMP-CRP
complex regulates transcription of these genes.
In conclusion, adenylate cyclase activity and competence in H. influenzae are
regulated by nutritional signals. It is proposed that cells may therefore transport
DNA for the nucleotides it contains, and that competence may be part of the hunger
response of H. influenzae and other bacteria to nutritional stress.
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