The galactose regulon of Escherichia coli

Characterization of a genetic locus essential for maltose-maltotriose utilization in Staphylococcus xylosus

A genetic locus from Staphylococcus xylosus involved in maltose-maltotriose utilization has been characterized. The chromosomal region was identified by screening a genomic library of S. xylosus in Escherichia coli for sucrose hydrolase activity. Nucleotide sequence analysis yielded two open reading frames (malR and malA) encoding proteins of 37.7 and 62.5 kDa, respectively. MalR was found to be homologous to the LacI-GalR family of transcriptional regulators, and MalA showed high similarity to yeast alpha-1,4-glucosidases and bacterial alpha-1,6-glucosidases. Inactivation of malA in the genome of S. xylosus led to a maltose-maltotriose-negative phenotype. In cell extracts of the mutant, virtually no glucose release from maltose and short maltodextrins was detectable. Inactivation of malA in a sucrose-6-phosphate hydrolase-deficient S. xylosus strain resulted in the complete loss of the residual sucrose hydrolase activity. The MalA enzyme has a clear preference for maltose but is also able to release glucose from short maltosaccharides. It cannot cleave isomaltose. Therefore, malA encodes an alpha-1,4-glucosidase or maltase, which also liberates glucose from sucrose. Subcloning experiments indicated that malA does not possess its own promoter and is cotranscribed with malR. Its expression could not be stimulated when maltose was added to the growth medium. Chromosomal inactivation of malR led to reduced maltose utilization, although alpha-glucosidase activity in the malR mutant was slightly higher than in the wild type. In the mutant strain, maltose uptake was reduced and inducibility of the transport activity was partially lost. It seems that MalR participates in the regulation of the gene(s) for maltose transport and is needed for their full expression. Thus, the malRA genes constitute an essential genetic locus for maltosaccharide utilization in S. xylosus

A chromosomal cluster of genes encoding ADP-glucose synthetase, glycogen synthase and phosphoglucomutase in Agrobacterium tumefaciens

A chromosomal region from Agrobacterium tumefaciens that complements exoC (pgm) mutations was cloned and sequenced. A cluster of three open reading frames (ORF1, ORF2 and ORF3) was identified. These genes are oriented in the same direction and are involved in the synthesis of glycogen and other polysaccharides. ORF1 encodes a 420-amino-acid (aa) protein with 55.9% homology to Escherichia coli GlgC (ADP-glucose synthetase, EC 2.7.7.27). ORF2 encodes a 480-aa protein with 42.2% homology to E. coli GlgA (glycogen synthase, EC 2.4.1.21). Based on Tn5 mutagenesis and protein homology, ORF3 was identified as the structural gene encoding phosphoglucomutase (Pgm, EC 2.7.5.1). ORF3 encodes a 542-aa protein with 52.6% homology to rabbit Pgm. There is no significant homology (less than 20%) to the Xanthomonas campestris XanA protein, which displays phosphomannomutase (Pmm) and Pgm activities [Koplin et al., J. Bacteriol 174 (1992) 191-199]. An A. tumefaciens pgm::Tn5 mutant retains Pmm activity.

Between feast and famine: endogenous inducer synthesis in the adaptation of Escherichia coli to growth with limiting carbohydrates

Escherichia coli adapted to growth with low carbohydrate concentrations bypassed the requirement for exogenous inducer with at least three well-studied sugar regulons. Induction of mgl and gal genes became independent of added galactose in bacteria approaching stationary phase or during continuous culture with micromolar glucose in the medium. Bacteria became independent of exogenous induction because endogenous galactose and cyclic AMP (cAMP) pools were sufficient for high expression of mgl and gal genes under glucose limitation. Limitation-stimulated induction of mgl was dependent on a functional galETK operon for synthesis of the inducer galactose. Intracellular galactose levels were maximal not during starvation (or slow steady-state growth rates approaching starvation) but at fast growth rates with micromolar glucose. The extent of mgl/gal induction correlated better with inducer availability than with cAMP concentrations under all conditions tested. Endogenous inducer accumulation represents an adaptation to low-nutrient environments, leading to derepression of high-affinity transport systems like Mgl essential for bacterial competitiveness at low nutrient concentrations.