Characterization of urease from the phototrophic bacteriumRhodobacter capsulatus E1F1

Seipin is a discrete homooligomer

Seipin is a transmembrane protein that resides in the endoplasmic reticulum and concentrates at junctions between the ER and cytosolic lipid droplets. Mutations in the human seipin gene, including the missense mutation A212P, lead to congenital generalized lipodystrophy (CGL), characterized by the lack of normal adipose tissue and accumulation of fat in liver and muscles. In both yeast and CGL patient fibroblasts, seipin is required for normal lipid droplet morphology; in its absence droplets appear to bud abnormally from the ER. Here we report the first purification and physical characterization of seipin. Yeast seipin is in a large discrete protein complex. Affinity purification demonstrated that seipin is the main if not exclusive protein in the complex. Detergent sucrose gradients in H(2)O, and D(2)O and gel filtration were used to determine the size of the seipin complex and account for detergent binding. Both seipin-myc13 (seipin fused to 13 tandem copies of the myc epitope) expressed from the endogenous promoter and overexpressed seipin-mCherry form ∼500 kDa proteins consisting of about 9 copies of seipin. The yeast orthologue of the human A212P allele forms only smaller complexes and is unstable; we hypothesize that this accounts for its null phenotype in humans. Seipin appears as a toroid by negative staining electron microscopy. We speculate that seipin plays at least a structural role in organizing droplets or in communication between droplets and ER.

Enzymatic, immunological and phylogenetic characterization of Brucella suis urease

BACKGROUND

The sequenced genomes of the Brucella spp. have two urease operons, ure-1 and ure-2, but there is evidence that only one is responsible for encoding an active urease. The present work describes the purification and the enzymatic and phylogenomic characterization of urease from Brucella suis strain 1330. Additionally, the urease reactivity of sera from patients diagnosed with brucellosis was examined.

RESULTS

Urease encoded by the ure-1 operon of Brucella suis strain 1330 was purified to homogeneity using ion exchange and hydrophobic interaction chromatographies. The urease was purified 51-fold with a recovery of 12% of the enzyme activity and 0.24% of the total protein. The enzyme had an isoelectric point of 5, and showed optimal activity at pH 7.0 and 28-35 degrees C. The purified enzyme exhibited a Michaelis-Menten saturation kinetics with a Km of 5.60 +/- 0.69 mM. Hydroxyurea and thiourea are competitive inhibitors of the enzyme with Ki of 1.04 +/- 0.31 mM and 26.12 +/- 2.30 mM, respectively. Acetohydroxamic acid also inhibits the enzyme in a competitive way. The molecular weight estimated for the native enzyme was between 130-135 kDa by gel filtration chromatography and 157 +/- 7 kDa using 5-10% polyacrylamide gradient non-denaturing gel. Only three subunits in SDS-PAGE were identified: two small subunits of 14,000 Da and 15,500 Da, and a major subunit of 66,000 Da. The amino terminal sequence of the purified large subunit corresponded to the predicted amino acid sequence encoded by ureC1. The UreC1 subunit was recognized by sera from patients with acute and chronic brucellosis. By phylogenetic and cluster structure analyses, ureC1 was related to the ureC typically present in the Rhizobiales; in contrast, the ureC2 encoded in the ure-2 operon is more related to distant species.

CONCLUSION

We have for the first time purified and characterized an active urease from B. suis. The enzyme was characterized at the kinetic, immunological and phylogenetic levels. Our results confirm that the active urease of B. suis is a product of ure-1 operon.

Urea utilization in the phototrophic bacterium Rhodobacter capsulatus is regulated by the transcriptional activator NtrC

The phototrophic nonsulfur purple bacterium Rhodobacter capsulatus can use urea as a sole source of nitrogen. Three transposon Tn5-induced mutations (Xan-9, Xan-10, and Xan-19), which led to a Ure(-) phenotype, were mapped to the ureF and ureC genes, whereas two other Tn5 insertions (Xan-20 and Xan-22) were located within the ntrC and ntrB genes, respectively. As in Klebsiella aerogenes and other bacteria, the genes encoding urease (ureABC) and the genes required for assembly of the nickel metallocenter (ureD and ureEFG) are clustered in R. capsulatus (ureDABC-orf136-ureEFG). No homologues of Orf136 were found in the databases, and mutational analysis demonstrated that orf136 is not essential for urease activity or growth on urea. Analysis of a ureDA-lacZ fusion showed that maximum expression of the ure genes occurred under nitrogen-limiting conditions (e.g., serine or urea as the sole nitrogen source), but ure gene expression was not substrate (urea) inducible. Expression of the ure genes was strictly dependent on NtrC, whereas sigma(54) was not essential for urease activity. Expression of the ure genes was lower (by a factor of 3.5) in the presence of ammonium than under nitrogen-limiting conditions, but significant transcription was also observed in the presence of ammonium, approximately 10-fold higher than in an ntrC mutant background. Thus, ure gene expression in the presence of ammonium also requires NtrC. Footprint analyses demonstrated binding of NtrC to tandem binding sites upstream of the ureD promoter. Phosphorylation of NtrC increased DNA binding by at least eightfold. Although urea is effectively used as a nitrogen source in an NtrC-dependent manner, nitrogenase activity was not repressed by urea.