Regulation of nitrogen metabolism genes by nifA gene product in Klebsiella pneumoniae

The Klebsiella pneumoniae nifA gene product, which is known to activate expression of the nitrogen fixation (nif) structural genes, is shown here also to be able to substitute for the product of the gene glnG (ntrC) in the regulation of other nitrogen metabolism genes. An evolutionary relationship between the nifA and glnG genes is suggested.     

Sinorhizobium meliloti nifA gene exerts a pleiotropic effect on nodulation through the enhanced plant defense response

Sinorhizobium meliloti nifA gene is required for the expression of a bunch of nif and fix genes. Here, we report its pleiotropic effects on the nodule formation. Compared with wild type strain, nifA mutant sig- nificantly reduced nodule suppression rate in split-root system. The plants inoculated with mutant strain produced lower amount of daidzein and less necrotic cells on their roots. In addition, the defense genes failed to be evoked by nifA mutant at the early nodulation stage. These findings indicated that host defense response was one of the mechanisms mediated by nifA gene to regulate nodule formation during symbiosis. Even though nifA mutant could increase the number of nodules in host plant, it synthesized lower Nod factors than wild type. This suggested that nifA gene mediated multiple and diverse instances in nodulation formation.     

肺炎克雷伯氏菌荚膜多糖的免疫活性研究

利用临床分离的肺炎克雷伯氏菌致病株制备其荚膜多糖（CPS），对CPS的免疫活性进行了研究。结果显示，CPS具有很强的免疫原性，以其免疫家兔制备的多克隆抗体效价达1：32000，且对CPS具有高度特异性；体外免疫活性实验表明，CPS对细胞免疫具有双向调节作用，即低浓度CPS对小鼠脾细胞增殖具有显著的促进作用，而高浓度CPS对小鼠脾细胞增殖具有显著的抑制作用；溶血空斑实验结果显示，CPS对体液免疫刺激作用显著（P〈0．01）；同时CPS能激活巨噬细胞．促进细胞因子TNF-α的生成。     

ATP sulphurylase activity of the nodP and nodQ gene products of Rhizobium meliloti

The symbiotic bacterium Rhizobium meliloti stimulates alfalfa (Medicago sativa L.) roots to undergo morphogenesis and form nitrogen-fixing nodules. It has been proposed that the bacterial genes nodABC, common to all Rhizobium, are required for synthesis of an oligosaccharide factor, which is converted to a sulphated form (NodRm-1) by the products of the R. meliloti-specific genes nodH and nodQ1-5; NodRm-1 elicits host-specific plant responses. Previously we have shown that the nodP gene is homologous to a segment of the Escherichia coli genome; when we cloned this E. coli fragment we found that it mapped near 59 minutes, corresponding to the cysDNC locus. The genes cysD and cysN encode proteins that catalyse the synthesis of adenosine 5'-phosphosulphate, the first step in the activation of inorganic sulphate. Here we demonstrate that nodP and nodQ correspond to cysD and cysN, and that their proteins have ATP sulphurylase activity both in vivo and in vitro. We propose that nodP and nodQ synthesize an activated sulphate that is an intermediate in the formation of the alfalfa-specific sulphated nodRm-1 factor.     

A haemoprotein with kinase activity encoded by the oxygen sensor of Rhizobium meliloti.

The expression of the nitrogen-fixation genes of Rhizobium meliloti is controlled by oxygen. These genes are induced when the free oxygen concentration is reduced to microaerobic levels. Two regulator proteins, FixL and FixJ, initiate the oxygen-response cascade, and the genes that encode them have been cloned. The fixL product seems to be a transmembrane sensor that modulates the activity of the fixJ product, a cytoplasmic regulator. FixL and FixJ are homologous to a family of bacterial two-component regulators, for which the mode of signal transduction is phosphorylation. We report here the purification of both FixJ and a soluble truncated FixL (FixL*), overproduced from a single plasmid construct. FixL* catalyses its own phosphorylation and the transfer of the gamma-phosphate of ATP to Fix J. The resulting FixJ-phosphate linkage is sensitive to base, as are the aspartyl phosphates of homologous systems. Visible spectra of purified FixL* show that it is an oxygen-binding haemoprotein. We propose that FixL senses oxygen through its haem moiety and transduces this signal by controlling the phosphorylation of FixJ.     

克雷伯氏肺炎杆菌生产1,3-丙二醇代谢途径中的酶活变化

考察了不同条件下克雷伯氏肺炎杆菌(Klebsiella.pneumoniae)合成1,3-丙二醇(1,3-PD)的3种关键酶(甘油脱氢酶(GDH)、1,3-PD氧化还原酶(PDOR)及甘油脱水酶(GDHz))的表观酶活变化。结果显示:这3种酶的酶活变化与Klebsiella.pneumoniae的1,3-PD代谢不完全相关。用SDS-PAGE电泳分析上述不同发酵条件下酶活变化,结果显示不同于3种酶的蛋白条带,Mr约为4.0×104位置的蛋白条带有明显的变化,这可能与菌体代谢过程相关,结论有待进一步深入研究。