The transcription factor NtcA is a global regulator of nitrogen homeostasis in cyanobacteria. It thus positively regulates the expression of genes related to nitrogen assimilation such as glnA (which encodes glutamine synthetase) and ntcA itself in response to nitrogen shortage or depletion. The binding of NtcA to the glnA and ntcA promoters of Synechococcus sp. PCC 7942 in vitro now has been shown to be enhanced by 2-oxoglutarate. In vitro analysis of gene transcription also revealed that the interaction of NtcA with its promoter element was not sufficient for activation of transcription, and 2-oxoglutarate was required for transcriptional initiation by NtcA. Given that the intracellular concentration of 2-oxoglutarate is inversely related to nitrogen availability, it is proposed that this metabolite functions as a signaling molecule that transmits information on cellular nitrogen status to NtcA and thereby regulates the transcription of genes related to nitrogen assimilation in cyanobacteria.regulatory factor ͉ cyanobacteria ͉ RNA polymerase ͉ nitrogen assimilation
Aspergillus nidulans possessed an ␣-glucosidase with strong transglycosylation activity. The enzyme, designated ␣-glucosidase B (AgdB), was purified and characterized. AgdB was a heterodimeric protein comprising 74-and 55-kDa subunits and catalyzed hydrolysis of maltose along with formation of isomaltose and panose. Approximately 50% of maltose was converted to isomaltose, panose, and other minor transglycosylation products by AgdB, even at low maltose concentrations. The agdB gene was cloned and sequenced. The gene comprised 3,055 bp, interrupted by three short introns, and encoded a polypeptide of 955 amino acids. The deduced amino acid sequence contained the chemically determined N-terminal and internal amino acid sequences of the 74-and 55-kDa subunits. This implies that AgdB is synthesized as a single polypeptide precursor. AgdB showed low but overall sequence homology to ␣-glucosidases of glycosyl hydrolase family 31. However, AgdB was phylogenetically distinct from any other ␣-glucosidases. We propose here that AgdB is a novel ␣-glucosidase with unusually strong transglycosylation activity.␣-Glucosidases (EC 3.2.1.20) catalyze liberation of glucose from nonreducing ends of ␣-glucosides, ␣-linked oligosaccharides, and ␣-glucans. They show diverse substrate specificities; some prefer ␣-linked di-, oligo-, and/or polyglucans, while others preferentially hydrolyze heterogeneous substrates such as aryl glucosides and sucrose (1, 5). Theoretically ␣-glucosidase is capable of catalyzing transglycosylation, since it is a retaining glycosyl hydrolase (GH) (2), and some ␣-glucosidases indeed exhibit clear transglycosylation activity. For example, Aspergillus niger ␣-glucosidase catalyzes formation of ␣-1,6 glucosidic linkages in addition to hydrolysis, resulting in production of isomaltose (6-O-␣-D-glucopyranosyl-D-glucopyranose) and panose (6-O-␣-glucopyranosyl-maltose) from maltose (3,15,21). Buckwheat ␣-glucosidase produces kojibiose (2-O-␣-glucosyl-glucose), nigerose (3-O-␣-glucosyl-glucose), maltose, and isomaltose from soluble starch (1), and ␣-glucosidases from Bacillus stearothermophilus and brewer's yeast produce oligosaccharides consisting of ␣-1,3, ␣-1,4, and ␣-1,6 linkages (13). Transglycosylation activity of the ␣-glucosidases has been applied in industries to produce isomaltooligosaccharides and also to conjugate sugars to biologically useful materials, aiming to improve their chemical properties and physiological functions (18, 33).The main physiological role of most exo-type glycosidases such as ␣-glucosidase is to produce monosaccharides that are utilized as carbon and energy sources. However, transglycosylation activities of exo-type glycosidases sometimes play physiologically important roles in gene regulation involved in carbohydrate utilization. A well-known example is induction of the lac operon in Escherichia coli. The physiological inducer of the operon, allolactose (6-O--D-galactopyranosyl-D-glucose), is synthesized from lactose by transglycosylation activity of -galactosidase encoded by ...
SummaryThe chromosome of the cyanobacterium Synechococcus sp. PCC7942 contains at least one group 1 (rpoD1) and three group 2 (rpoD2, rpoD3 and rpoD4) sigma factor genes. In this study, we have analysed the structure of rpoD3 and rpoD4 and have shown that these genes are dispensable for growth at normal physiological conditions. An RNA polymerase core enzyme of the cyanobacterial strain was puri®ed, reconstituted with the recombinant sigma factors (the rpoD1, rpoD3 and rpoD4 gene products), and the resultant holoenzymes were examined in vitro for transcription speci®city. All of the holoenzymes recognized canonical promoters of Escherichia coli as well as cyanobacterial rrnA, cpcB1A1 P1a and rpoD1 promoters, although the three holoenzymes had some preference for speci®c promoters. These results suggest that group 1 as well as group 2 sigma factors of cyanobacteria may direct transcription initiation from the eubacterial consensus-type promoters containing the Pribnow À10 element, and we postulate that speci®city crosstalk is a common characteristic among eubacterial group 1 and group 2 sigma factors. Phylogenetic analyses revealed that most group 2 sigma factors were positioned in one of four distinct clusters. The implication of the phylogenetic tree is also discussed in this paper.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.