|
|
|
|
|
|
Role of the Murein Precursor UDP-N-Acetylmuramyl-L-Ala-  -D-Glu- meso-Diaminopimelic Acid-D-Ala-D-Ala in Repression of ß-Lactamase Induction in Cell Division Mutants.Tsuyoshi Uehara, 2002.Certain ß-lactam antibiotics induce the chromosomal ampC ß-lactamase of many gram-negative bacteria . The natural inducer, though not yet unequivocally identified, is a cell wall breakdown product which enters the cell via the AmpG permease component of the murein recycling pathway . Surprisingly, it has been reported that ß-lactamase is not induced by cefoxitin in the absence of FtsZ, which is required for cell division, or in the absence of penicillin-binding protein 2 (PBP2), which is required for cell elongation . Since these results remain unexplained, we examined an ftsZ mutant and other cell division mutants (ftsA, ftsQ, and ftsI) and a PBP2 mutant for induction of ß-lactamase . In all mutants, ß-lactamase was not induced by cefoxitin, which confirms the initial reports . The murein precursor, UDP-N-acetylmuramyl-L-Ala- -D-Glu-meso-diaminopimelic acid-D-Ala-D-Ala (UDP-MurNAc-pentapeptide), has been shown to serve as a corepressor with AmpR to repress ß-lactamase expression in vitro . Our results suggest that ß-lactamase is not induced because the fts mutants contain a greatly increased amount of corepressor which the inducer cannot displace . In the PBP2(Ts) mutant, in addition to accumulation of corepressor, cell wall turnover and recycling were greatly reduced so that little or no inducer was available . Hence, in both cases, a high ratio of repressor to inducer presumably prevents induction .
Characterization of Helicobacter pylori Nickel Metabolism Accessory Proteins Needed for Maturation of both Urease and Hydrogenase. Nalini Mehta, 2003.Previous studies demonstrated that two accessory proteins, HypA and HypB, play a role in nickel-dependent maturation of both hydrogenase and urease in Helicobacter pylori . Here, the two proteins were purified and characterized . HypA bound two Ni2+ ions per dimer with positive cooperativity (Hill coefficient, approximately 2.0) . The dissociation constants K1 and K2 for Ni2+ were 58 and 1.3 µM, respectively . Studies on purified site-directed mutant proteins in each of the five histidine residues within HypA, revealed that only one histidine residue (His2) is vital for nickel binding . Nuclear magnetic resonance analysis showed that this purified mutant version (H2A) was similar in structure to that of the wild-type HypA protein . A chromosomal site-directed mutant of hypA (in the codon for His2) lacked hydrogenase activity and possessed only 2% of the wild-type urease activity . Purified HypB had a GTPase activity of 5 nmol of GTP hydrolyzed per nmol of HypB per min . Site-directed mutagenesis within the lysine residue in the conserved GTP-binding motif of HypB (Lys59) nearly abolished the GTPase activity of the mutant protein (K59A) . In native solution, both HypA and HypB exist as homodimers with molecular masses of 25.8 and 52.4 kDa, respectively . However, a 1:1 molar mixture of HypA plus HypB gave rise to a 43.6-kDa species composed of both proteins . A 43-kDa heterodimeric HypA-HypB complex was also detected by cross-linking . The cross-linked adduct was still observed in the presence of 0.5 mM GTP or 1 µM nickel or when the mutant version of HypA (altered in His2) and HypB (altered in Lys59) were tested . Individually, HypA and HypB formed homodimeric cross-linked adducts . An interaction between HypA and the Hp0868 protein (encoded by the gene downstream of hypA) could not be detected via cross-linking, although such an interaction was predicted by yeast two-hybrid studies . In addition, the phenotype of an insertional mutation within the Hp0868 gene indicated that its presence is not critical for either the urease or the hydrogenase activity . Glucose Metabolism in Lactococcus lactis MG1363 under Different Aeration Conditions: Requirement of Acetate To Sustain Growth under Microaerobic Conditions. Mikkel Nordkvist, 2003.Lactococcus lactis subsp . lactis MG1363 was grown in batch cultures on a defined medium with glucose as the energy source under different aeration conditions, namely, anaerobic conditions, aerobic conditions, and microaerobic conditions with a dissolved oxygen tension of 5% (when saturation with air was used as the reference) . The maximum specific growth rate was high (0.78 to 0.91 h-1) under all aeration conditions but decreased with increasing aeration, and more than 90% of the glucose was converted to lactate . However, a shift in by-product formation was observed . Increasing aeration resulted in acetate, CO2, and acetoin replacing formate and ethanol as end products . Under microaerobic conditions, growth came to a gradual halt, although more than 60% of the glucose was still left . A decline in growth was not observed during microaerobic cultivation when acetate was added to the medium . We hypothesize that the decline in growth was due to a lack of acetyl coenzyme A (acetyl-CoA) needed for fatty acid synthesis since acetyl-CoA can be synthesized from acetate by means of acetate kinase and phosphotransacetylase activities .
|
© 2005
Transgalactic Ltd (manufacturer of Bioscreen C software) |
Privacy Statement | P.O. Box
1393, 00101 Helsinki, Finland,
Last modified: May 25, 2005
| ||||||
