Shewanella oneidensis MR-1 has the metabolic capacity to grow anaerobically using Fe(III) as a terminal electron acceptor. Growth under these conditions results in the de novo synthesis of a number of periplasmic c-type cytochromes, many of which are multiheme in nature and are thought to be involved in the Fe(III) respiratory process. To begin a biochemical study of these complex cytochromes, the mtrA gene that encodes an approximate 32-kDa periplasmic decaheme cytochrome has been heterologously expressed in Escherichia coli. Co-expression of mtrA with a plasmid that contains cytochrome c maturation genes leads to the assembly of a correctly targeted holoprotein, which covalently binds ten hemes. The recombinant MtrA protein has been characterized by magnetic circular dichroism, which shows that all ten hemes have bis-histidine axial ligation. EPR spectroscopy detected only eight of these hemes, all of which are low spin and provides evidence for a spin-coupled pair of hemes in the oxidized state. Redox titrations of MtrA have been carried out with optical- and EPR-monitored methods, and the hemes are shown to reduce over the potential range –100 to –400 mV. In intact cells of E. coli, MtrA is shown to obtain electrons from the host electron transport chain and pass these onto host oxidoreductases or a range of soluble Fe(III) species. This demonstrates the promiscuous nature of this decaheme cytochrome and its potential to serve as a soluble Fe(III) reductase in intact cells.

PII‐like signal transduction proteins, which respond to the nitrogen status via covalent modification and signal the carbon status through the binding of 2‐oxoglutarate, have been implicated in the regulation of nitrogen fixation in several diazotrophs. The NIFL–NIFA two‐component regulatory system, which integrates metabolic signals to fine‐tune regulation of nitrogenase synthesis in Azotobacter vinelandii, is a potential target for PII‐mediated signal transduction. Here we demonstrate that the inhibitory activity of the A.vinelandii NIFL protein is stimulated by interaction with the non‐uridylylated form of PII‐like regulatory proteins. We also observe that the NIFL–NIFA system is directly responsive to 2‐oxoglutarate. We propose that the PII protein signals the nitrogen status by interaction with the NIFL–NIFA system under conditions of nitrogen excess, and that the inhibitory activity of NIFL is relieved by elevated levels of 2‐oxoglutarate when PII is uridylylated under conditions of nitrogen limitation. Our observations suggest a model for signal transduction to the NIFL–NIFA system in response to carbon and nitrogen status which is clearly distinct from that suggested from studies on other diazotrophs.

UDP-glucose: glycoprotein glucosyltransferase (UGGT) is a key player in the quality control mechanism (ER-QC) that newly synthesized glycoproteins undergo in the ER. It has been shown that the UGGT Arabidopsis orthologue is involved in ER-QC; however, its role in plant physiology remains unclear. Here, we show that two mutant alleles in the At1g71220 locus have none or reduced UGGT activity. In wild type plants, the AtUGGT transcript levels increased upon activation of the unfolded protein response (UPR). Interestingly, mutants in AtUGGT exhibited an endogenous up–regulation of genes that are UPR targets. In addition, mutants in AtUGGT showed a 30 % reduction in the incorporation of UDP-Glucose into the ER suggesting that this enzyme drives the uptake of this substrate for the CNX/CRT cycle. Plants deficient in UGGT exhibited a delayed growth rate of the primary root and rosette as well as an alteration in the number of leaves. These mutants are more sensitive to pathogen attack as well as heat, salt, and UPR-inducing stressors. Additionally, the plants showed impairment in the establishment of systemic acquired resistance (SAR). These results show that a lack of UGGT activity alters plant vegetative development and impairs the response to several abiotic and biotic stresses. Moreover, our results uncover an unexpected role of UGGT in the incorporation of UDP-Glucose into the ER lumen in Arabidopsis thaliana.

Screening of Arabidopsis mutants with altered seed mucilage allowed identification of UUAT1, a Golgi-localized protein that transports UDP-glucuronic acid and p