Epithelial cells detect functional type III secretion system of enteropathogenic Escherichia coli through a novel NF-κB signaling pathway.

Enteropathogenic Escherichia coli (EPEC), a common cause of infant diarrhea, is associated with high risk of mortality in developing countries. The primary niche of infecting EPEC is the apical surface of intestinal epithelial cells. EPEC employs a type three secretion system (TTSS) to inject the host cells with dozens of effector proteins, which facilitate attachment to these cells and successful colonization. Here we show that EPEC elicit strong NF-κB activation in infected host cells. Furthermore, the data indicate that active, pore-forming TTSS per se is necessary and sufficient for this NF-κB activation, regardless of any specific effector or protein translocation. Importantly, upon infection with wild type EPEC this NF-κB activation is antagonized by anti-NF-κB effectors, including NleB, NleC and NleE. Accordingly, this NF-κB activation is evident only in cells infected with EPEC mutants deleted of nleB, nleC, and nleE. The TTSS-dependent NF-κB activation involves a unique pathway, which is independent of TLRs and Nod1/2 and converges with other pathways at the level of TAK1 activation. Taken together, our results imply that epithelial cells have the capacity to sense the EPEC TTSS and activate NF-κB in response. Notably, EPEC antagonizes this capacity by delivering anti-NF-κB effectors into the infected cells.

The hierarchy of transition metal homeostasis: iron controls manganese accumulation in a unicellular cyanobacterium.

Iron and manganese are part of a small group of transition metals required for photosynthetic electron transport. Here, we present evidence for a functional link between iron and manganese homeostasis. In the unicellular cyanobacterium, Synechocystis sp. PCC 6803, Fe and Mn deprivation resulted in distinct modifications of the physiological status. The effect on growth and photosynthetic activity under Fe limitation were more severe than those observed under Mn limitation. Moreover, the intracellular elemental quotas of Fe and Mn were found to be linked. Fe limitation reduced the intracellular Mn quota. Mn limitation did not exert a reciprocal effect on Fe quotas. Microarray analysis comparing Mn and Fe limitation revealed a stark difference in the extent of the transcriptional response to the two limiting conditions, reflective of the physiological responses. The effects of Fe limitation on the transcriptional network are widespread while the effects on Mn limitation are highly specific. Our analysis also revealed an overlap in the transcriptional response of specific Fe and Mn transporters. This overlap provides a framework for explaining Fe limitation induced changes in Mn quotas.

Balancing photosynthetic electron flow is critical for cyanobacterial acclimation to nitrogen limitation.

Nitrogen limitation forces photosynthetic organisms to reallocate available nitrogen to essential functions. At the same time, it increases the probability of photo-damage by limiting the rate of energy-demanding metabolic processes, downstream of the photosynthetic apparatus. Non-diazotrophic cyanobacteria cope with this situation by decreasing the size of their phycobilisome antenna and by modifying their photosynthetic apparatus. These changes can serve two purposes: to provide extra amino-acids and to decrease excitation pressure. We examined the effects of nitrogen limitation on the form and function of the photosynthetic apparatus. Our aim was to study which of the two demands serve as the driving force for the remodeling of the photosynthetic apparatus, under different growth conditions. We found that a drastic reduction in light intensity allowed cells to maintain a more functional photosynthetic apparatus: the phycobilisome antenna was bigger, the activity of both photosystems was higher and the levels of photosystem (PS) proteins were higher. Pre-acclimating cells to Mn limitation, under which the activity of both PSI and PSII is diminished, results in a very similar response. The rate of PSII photoinhibition, in nitrogen limited cells, was found to be directly related to the activity of the photosynthetic apparatus. These data indicate that, under our experimental conditions, photo-damage avoidance was the more prominent determinant during the acclimation process. The combinations of limiting factors tested here is by no means artificial. Similar scenarios can take place under environmental conditions and should be taken into account when estimating nutrient limitations in nature.