The amount of ethylene produced

The amount of ethylene produced RAD001 price in early samples was similar in cultures with or without C10-HSL but, interestingly, a progressively decreased ethylene production was observed in the C10-HSL-treated culture, resulting in a 30% decrease of nitrogenase activity (data not shown). The progressive increase of the inhibitory effect of AHLs in acclimated cultures could perhaps be caused by the entry of the AHLs in the new generations of heterocysts, as the impermeability of the wall of mature heterocysts could prevent the penetration of the AHLs. Nonetheless it cannot be excluded that although AHLs

could enter through vegetative walls and spread along the filaments by the periplasmic space (Flores et al., 2006; Mariscal et al., 2007), entering in both mature and forming heterocysts, these molecules could only act at the molecular level in newly formed heterocysts. In that case the results observed would suggest a nonreversible inhibition of nitrogenase in very early stages at the level of either gene expression or its enzymatic activity. Because all tested AHLs showed inhibitory activity this website on nitrogen fixation mostly in newly formed heterocysts, to study possible effects at the level of expression of nitrogen metabolism genes, Northern blots were carried out to detect changes in expression of the dinitrogenase reductase

subunit gene (nifH) and fdxH, encoding a heterocyst-specific ferrodoxin that is a likely electron donor to dinitrogenase reductase (Razquin et al., 1995). No significant differences in the expression of either gene could be detected at 20 and 24 h after nitrogen step-down (no expression of nifH and fdxH

was detected at 0, 3 or 6 h) in total RNA extracted from C10-HSL-treated cultures when compared with control samples (Fig. 4). This indicates that the process of heterocyst differentiation proceeds normally in the presence of AHLs and therefore AHL inhibition could be affecting either the expression of other genes related to nitrogen fixation or be acting on nitrogenase-related genes at a post-transcriptional level. Finally, the strong inhibition of nitrogenase demonstrated for through all the AHLs tested and the cytotoxic effect of OC10-HSL in the presence of combined nitrogen represent novel biological activities of these signal molecules. The observation that antibiotics cannot easily reach the lethal concentrations in natural environments has led to a questioning of whether these molecules could act, in subinhibitory concentrations, as signal molecules (Davies, 2006; Linares et al., 2006). Low concentrations of several antibiotics can alter expression patterns of bacteria without any effect on growth rate (Davies et al., 2006), which resembles the mode of action of QS signals.

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