Effect of dimethylsulfoxide on derepression of nitrogenase in Spirillum lipoferum

A citrate-inducible gene, encoding a putative tricarboxylate transporter, is downregulated by the organic solvent DMSO in Agrobacterium tumefaciens

AIMS

To investigate the effects of the organic solvent dimethyl sulfoxide (DMSO) on the expression of a citrate-inducible gene, encoding a putative tricarboxylate transporter, in Agrobacterium tumefaciens.

METHODS AND RESULTS

By two-dimensional gel electrophoresis, we discovered a putative tricarboxylate transporter named ActC, whose expression was downregulated by DMSO. The expression of actC is also induced by tricarboxylates but not affected by other organic acids of the TCA cycle. Intriguingly, transcriptional activation of actC by citrate is compromised in the presence of DMSO. Furthermore, expression of actC is abolished by deletion of actDE, encoding a putative two-component regulatory system upstream of the actCBA gene cluster.

CONCLUSIONS

actC is a citrate-inducible gene that is repressed by DMSO and whose expression is likely regulated by a two-component system.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study provides useful information as to a potential DMSO-regulatory system of A. tumefaciens or other soil bacteria when encountering DMSO in nature. In addition, DMSO-regulated genes should be taken into account for studies in which bacterial cultures were treated with compounds dissolved in DMSO.

Isolation of a glutamate synthase (GOGAT)-negative, pleiotropically N utilization-defective mutant of Azospirillum brasilense: cloning and partial characterization of GOGAT structural gene

An Azospirillum brasilense mutant (N12) pleiotropically defective in the assimilation of nitrogenous compounds (Asm-) was isolated and found lacking in the glutamate synthase (GOGAT-). The glt (GOGAT) locus of A. brasilense was identified by isolating a broad-host-range pLAFR1 cosmid clone from a gene library of the bacterium that rectified Asm- and GOGAT- defects (full recovery of activities of the nitrogenase, the assimilatory nitrate and nitrite reductases, and the glutamate synthase). A 7.5-kb EcoRI fragment of the cosmid clone that also complemented N12 was partially sequenced to identify the open reading frame for the alpha-subunit of GOGAT. The amino acid sequences deduced from the partial nucleotide sequences of the glt locus of A. brasilense showed considerable homology with that of the alpha-subunit of GOGAT coded by the gltB gene of Escherichia coli. The genetic lesion of N12 was found within the gltB gene of A. brasilense. The gltB promoter of A. brasilense showed the presence of a consensus sigma-70-like recognition site (as in E. coli) in addition to potential NtrA-RNA polymerase, IHF, and NifA binding sites.

NADPH/NADH-dependent cold-labile glutamate dehydrogenase in Azospirillum brasilense. Purification and properties

A cold-labile glutamate dehydrogenase (GDH, EC 1.4.1.3) has been purified to homogeneity from the crude extracts of Azospirillum brasilense. The purified enzyme shows a dual coenzyme specificity, and both the NADPH and NADH-dependent activities are equally cold-sensitive. The enzyme is highly specific for the substrates 2-oxoglutarate and glutamate. Kinetic studies with GDH indicate that the enzyme is primarily designed to catalyse the reductive amination of 2-oxoglutarate. The NADP+-linked activity of GDH showed Km values 2.5 X 10(-4) M and 1.0 X 10(-2) M for 2-oxoglutarate and glutamate respectively. NAD+-linked activity of GDH could be demonstrated only for the amination of 2-oxoglutarate but not for the deamination of glutamate. The Lineweaver-Burk plot with ammonia as substrate for NADPH-dependent activity shows a biphasic curve, indicating two apparent Km values (0.38 mM and 100 mM) for ammonia; the same plot for NADH-dependent activity shows only one apparent Km value (66 mM) for ammonia. The NADPH-dependent activity shows an optimum pH from 8.5 to 8.6 in Tris/HCl buffer, whereas in potassium phosphate buffer the activity shows a plateau from pH 8.4 to 10.0. At high pH (greater than 9.5) amino acids in general strongly inhibit the reductive amination reaction by their competition with 2-oxoglutarate for the binding site on GDH. The native enzyme has a Mr = 285000 +/- 20000 and appears to be composed of six identical subunits of Mr = 48000 +/- 2000. The GDH level in A. brasilense is strongly regulated by the nitrogen source in the growth medium.

Identification of a phosphoenolpyruvate:fructose 1-phosphotransferase system in Azospirillum brasilense

An inducible phosphoenolpyruvate:fructose phosphotransferase system has been detected in Azospirillum brasilense, which requires a minimum of two components of the crude extracts for activity: (i) a soluble fraction (enzyme I) and (ii) a membrane fraction (enzyme II). The uninduced cells neither show any uptake of fructose nor express activity of either of these two enzyme fractions. C-1 of fructose is the site of phosphorylation. This phosphotransferase system does not accept glucose as a substrate for phosphorylation.