RAPISARDA Viviana Andrea
congresos y reuniones científicas
Unusual phosphate-dependent expression of Escherichia coli respiratory genes in stationary phase
Rosario (Santa Fe)
Congreso; V Congreso Argentino de MicrobiologĂ­a General; 2008
Institución organizadora:
Sociedad Argentina de MicrobiologĂ­a General (SAMIGE)
In Escherichia coli, the three types of respiratory components are: (1) dehydrogenases, which carry out the oxidation of organic substrates and feed electrons into mobile quinone pool, (2) quinones, which deliver reducing equivalents to the terminal oxidases, and (3) oxidases, which reduce the terminal electron acceptors. Bacteria alter the composition of the respiratory chain as part of its ability to adapt to different growth conditions. The amount of each component is strictly regulated to optimize the respiratory chain, and maintain the redox balance, according to the substrates present and the physiological needs of the cell. In response to growth arrest, the aerobic electron transport chain components are down-regulated. Previously, we have found that the ndh gene, encoding respiratory NADH dehydrogenase-2, was unusually expressed in late stationary phase when phosphate concentration was >37 mM. In the former conditions, we have also demonstrated that cells presented higher oxygen consumption rates, were more viable, and had a lower NADH/NAD+ ratio (similar to exponential) than cells grown in sufficient phosphate media (2 mM). Here, the expression of ndh and the other respiratory chain genes (nuoAB, sdhC, cyoA, cydA, and ubiC) was tested under different phosphate concentration up to 96 h. In addition, NADH and succinate dehydrogenase, and NADH oxidase activities were assayed. The presence of high phosphate concentration in the culture media maintains the dehydrogenase activities and respiratory genes expression for at least 96 h, excepting NADH oxidase activity and cyoA gene expression. In order to investigate if the phosphate-dependet expression was mediated by a global transcriptional factor, a comparative study of respiratory promotors was done. Then, ndh expression was assayed using strains deficient in FNR, IHF, ArcA, and Fis. Any of those regulators was the responsible of phosphate effect in stationary phase. The present results point that the salt composition of the minimal culture media should be carefully considered in the experimental design, especially when stationary phase events are studied. Our results also suggest that phosphate concentrations in stationary phase may act as a signal for the maintenance of aerobic metabolism by a not yet described mechanism. Moreover, the long-term effect of phosphate concentration on respiratory chain enzymes is promising to postulate an unknown role of the respiratory enzymes in late stationary phase.