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J M Cock

Publications and source records attributed to J M Cock.

23 records · Page 2Linked to original sources
PubMed1991

Regulation of glutamine synthetase genes in leaves of Phaseolus vulgaris.

Glutamine synthetase (GS) activity increased over three-fold in developing primary leaves of Phaseolus vulgaris L. This increase was shown to be the result of differential expression of three members of the GS gene family: gln-alpha and gln-beta, which encode cytosolic GS polypeptides, and gln-delta, which encodes the chloroplast-located GS. The gln-delta gene was the most highly expressed GS gene and was regulated in a complex manner with two different transcripts accumulating differentially during leaf development. This gene was expressed weakly in the dark and was induced strongly by light; this induction was shown not to be an indirect effect of photorespiration. In the long term, gln-delta showed increased expression in photorespiring compared with non-photorespiring leaves. However, in the short term, there was no induction of gln-delta following transfer of plants to photorespiratory conditions. These results suggest that regulation of gln-delta by photorespiration was the result of indirect, long-term effects on cellular metabolism. In general, in all these experiments, analysis of cytosolic versus chloroplastic GS polypeptides and of the GS isoenzyme profiles showed the same pattern of changes in abundance as that observed for the mRNAs suggesting that regulation of GS gene expression occurred primarily at the mRNA level. However, it was noteworthy that the delta isoenzyme remained at a high abundance in older leaves, grown in both light and dark, despite a decrease in abundance of gln-delta mRNA.

Blotting, Northern↗
PubMed1991

A nuclear gene with many introns encoding ammonium-inducible chloroplastic NADP-specific glutamate dehydrogenase(s) in Chlorella sorokiniana.

Chlorella sorokiniana possesses ammonium-inducible, chloroplastic, NADP-specific glutamate dehydrogenase (NADP-GDH) homo- or heterohexamers composed of alpha- and/or beta-subunits which were previously shown to derive from precursor protein(s) of identical size. From the present studies, data are consistent with these two subunits being encoded by a single nuclear gene. The NADP-GDH gene is greater than 7 kb in length due to the presence of at least 21 introns, an unusually large number for a eukaryotic microorganism. The exons, identified by comparison with sequences of NADP-GDH cDNA clones, include a region which is highly conserved among NADP-GDH genes. This region in the C. sorokiniana gene is 77% and 73% identical to the corresponding regions in the Escherichia coli and Neurospora crassa NADP-GDH genes, respectively. Seventeen independent NADP-GDH cDNA clones were analyzed by restriction mapping and partial sequencing, and no differences were detected among them. The longest cDNA was fused in frame with lacZ in a Bluescript vector and was expressed in E. coli as NADP-GDH antigen. During a 240 min induction period, under conditions in which both types of subunits were synthesized, only a single (2.2 kb) NADP-GDH mRNA band was detected on northern blots using cDNA probes from the highly conserved and 3'-untranslated regions. Collectively, these results are consistent with a single mRNA encoding a precursor-protein which is differentially processed to yield either an alpha- or beta-subunit.

Amino Acid Sequence↗
PubMed1990

Expression of glutamine synthetase genes in roots and nodules of Phaseolus vulgaris following changes in the ammonium supply and infection with various Rhizobium mutants.

In this paper we have examined whether the four glutamine synthetase (gln) genes, expressed in roots and nodules of Phaseolus vulgaris are substrate-inducible by ammonium. Manipulation of the ammonium pool in roots, through addition and removal of exogenous ammonium, did not elicit any changes in the abundances of the four mRNAs thus suggesting that the gln genes in roots of this legume are neither substrate-inducible by ammonium nor derepressed during nitrogen starvation. In nodules the effect of the ammonium supply on expression of the gln genes has been examined by growing nodules under argon/oxygen atmospheres, or with a number of Fix- Rhizobium mutants, and following addition of exogenous ammonium. The results of these experiments suggest that the expression of the gln-gamma gene, which is strongly induced during nodule development, is primarily under a developmental control. However nitrogen fixation appears to have a quantitative effect on expression of gln-gamma as the abundance of this mRNA is about 2 to 4-fold higher under nitrogen-fixing conditions. This effect could not be mimicked by addition of exogenous ammonium and moreover is not specific to the gln-gamma gene as mRNA from a leghaemoglobin gene was similarly affected. Taken together these results have failed to find an effect of ammonium on specifically inducing the expression of glutamine synthetase genes in roots and nodules of P. vulgaris.

Ammonia↗