Spx is a global transcriptional regulator that plays a key role in stress response [, , , , , ]. It acts as an "anti-alpha"factor and interacts with the RNA polymerase alpha subunit C-terminal domain. RNA polymerase is therefore prevented from responding to certain activator proteins such as ResD and ComA []. Spx is a negative regulator of competence []. It was previously known as YjbD [].
The SPX domain is named after SYG1/Pho81/XPR1 proteins. This 180 residue length domain is found at the amino terminus of a variety of proteins. In the yeast protein SYG1, the N terminus directly binds to the G- protein beta subunit and inhibits transduction of the mating pheromone signal []suggesting that all the members of this family are involved in G-protein associated signal transduction. The C-terminal of these proteins often have an EXS domain () [].The N-termini of several proteins involved in the regulation of phosphate transport, including the putative phosphate level sensors PHO81 from Saccharomyces cerevisiae and NUC-2 from Neurospora crassa, are also members of this family [, ]. NUC-2 contains several ankyrin repeats ().Several members of this family are the XPR1 proteins: the xenotropic and polytropic retrovirus receptor confers susceptibility to infection with Murine leukemia virus (MLV) []. The similarity between SYG1, phosphate regulators and XPR1 sequences has been previously noted, as has the additional similarity to several predicted proteins, of unknown function, from Drosophila melanogaster, Arabidopsis thaliana, Caenorhabditis elegans, Schizosaccharomyces pombe, and Saccharomyces cerevisiae [, ]. In addition, given the similarities between XPR1 and SYG1 and phosphate regulatory proteins, it has been proposed that XPR1 might be involved in G-protein associated signal transduction [, , ]and may itself function as a phosphate sensor [].
This domain has been named the SPX domain after (Syg1, Pho81 and XPR1). The SPX domain is found at the amino terminus of a variety of proteins. Proteins containing this domain, mostly found in plants, contains a C-terminal MFS domain (major facilitator superfamily), suggesting a function as a secondary transporter. The function of this N-terminal region is unclear, although it might be involved in regulating transport [].
This entry represents a group of plant SPX domain-containing proteins, including AtSPX1-6 from Arabidopsis. AtSPX1 () and AtSPX2 () have a cellular Pi-dependent inhibitory effect on Phosphate Starvation Response 1 (PHR1) []. Their rice homologue, OsSPX1 (), may play an important role in linking cold stress and Pi starvation signal transduction pathways [].
This region has been named the SPX domain after (Syg1, Pho81 and XPR1). The domain is found at the amino terminus of a variety of proteins. The PHO1 gene family conserved in plants is involved in a variety of processes, most notably the transport of inorganic phosphate from the root to the shoot of the plant and mediating the response to low levels of inorganic phosphate []. PHO1 may play roles in stress response as well as the stomatal response to abscisic acid [].
The YhfH-like protein family includes the Bacillus subtilis YhfH protein , which is functionally uncharacterised. Its expression is repressed by the Spx paralogue MgsR, which regulates genes involved in stress response [].
Characterized members of this family include Spx and MgsR from Bacillus subtilis. Spx is a global transcription regulator that plays a key role in stress response and exerts either positive or negative regulation of genes. Induces the expression of a large number of genes in response to a variety of stress conditions, such as disulfide, heat and cell wall stress. It interacts with RNA polymerase [, ]. MgsR (modulator of the general stress response, also called YqgZ) provides a second level of regulation for more than a third of the proteins in the B. subtilis general stress regulon controlled by Sigma-B [].
MecA enables the recognition and targeting of unfolded and aggregated proteins to the ClpC protease or to other proteins involved in proteolysis. It acts negatively in the development of competence by binding ComK and recruiting it to the ClpCP protease. When overexpressed, it inhibits sporulation. It is also involved in Spx degradation by ClpC []. This entry represents the C terminus region of MecA.
This entry represents a group of proteins from Firmicutes, including ClpXP adapter protein SpxH, also known as YjbH in Bacillus subtilis. This protein is required for efficient degradation of the RNA polymerase-binding transcription factor Spx by the protease ClpXP under non-stress conditions. It is organised into a DsbA-like thioredoxin domain, a linker and a C-terminal domain reminiscent of the winged helix-turn-helix fold [, , ].
Several bacterial taxon have a chromosomal resistance system, encoded by the ars operon, for the detoxification of arsenate, arsenite, and antimonite []. This system transports arsenite and antimonite out of the cell. The pump is composed of two polypeptides, the products of the arsA and arsB genes. This two-subunit enzyme produces resistance to arsenite and antimonite. Arsenate, however, must first be reduced to arsenite before it is extruded. A third gene, arsC, expands the substrate specificity to allow for arsenate pumping and resistance. ArsC is an approximately 150-residue arsenate reductase that uses reduced glutathione (GSH) to convert arsenate to arsenite with a redox active cysteine residue in the active site. ArsC forms an active quaternary complex with GSH, arsenate, and glutaredoxin 1 (Grx1). The three ligands must be present simultaneously for reduction to occur [].The arsC family also comprises the Spx proteins which are GRAM-positive bacterial transcription factors that regulate the transcription of multiple genes in response to disulphide stress [, ].The arsC protein structure has been solved []. It belongs to the thioredoxin superfamily fold which is defined by a β-sheet core surrounded by α-helices. The active cysteine residue of ArsC is located in the loop between the first β-strand and the first helix, which is also conserved in the Spx protein and its homologues.
The EXS domain is named after ERD1/XPR1/SYG1 and proteins containing this motif include the C-terminal of the SYG1 G-protein associated signal transduction protein from Saccharomyces cerevisiae, and sequences that are thought to be Murine leukemia virus (MLV) receptors (XPR1. The N-terminal of these proteins often have an SPX domain () [].While the N-terminal is thought to be involved in signal transduction, the role of the C-terminal is not known. This region of similarity contains several predicted transmembrane helices. This family also includes the ERD1 (ERD: ER retention defective) S. cerevisiae proteins. ERD1 proteins are involved in the localization of endogenous endoplasmic reticulum (ER) proteins. Erd1 null mutants secrete such proteins even though they possess the C-terminal HDEL ER lumen localization label sequence. In addition, null mutants also exhibit defects in the Golgi-dependent processing of several glycoproteins, which led to the suggestion that the sorting of luminal ER proteins actually occurs in the Golgi, with subsequent return of these proteins to the ER via `salvage' vesicles [].
Competence is the ability of a cell to take up exogenous DNA from its environment, resulting in transformation. It is widespread among bacteria and is probably an important mechanism for the horizontal transfer of genes. DNA usually becomes available by the death and lysis of other cells. Competent bacteria use components of extracellular filaments called type 4 pili to create pores in their membranes and pull DNA through the pores into the cytoplasm. This process, including the development of competence and the expression of the uptake machinery, is regulated in response to cell-cell signalling and/or nutritional conditions [].This family contains several bacterial MecA proteins. In complex media competence development is poor, and there is little or no expression of late competence genes. Overexpression of MecA inhibits comG transcription [, , ].MecA enables the recognition and targeting of unfolded and aggregated proteins to the ClpC protease or to other proteins involved in proteolysis. It acts negatively in the development of competence by binding ComK and recruiting it to the ClpCP protease. When overexpressed, it inhibits sporulation. It is also involved in Spx degradation by ClpC [].
