Differential processing of homologues of the small subunit of ADP-glucose pyrophosphorylase from barley (Hordeum vulgare) tissues

Identification of the ADP-glucose pyrophosphorylase isoforms essential for starch synthesis in the leaf and seed endosperm of rice (Oryza sativa L.)

ADP-glucose pyrophosphorylase (AGP) catalyzes the first committed step of starch biosynthesis in higher plants. To identify AGP isoforms essential for this biosynthetic process in sink and source tissues of rice plants, we analyzed the rice AGP gene family which consists of two genes, OsAGPS1 and OsAGPS2, encoding small subunits (SSU) and four genes, OsAGPL1, OsAGPL2, OsAGPL3 and OsAGPL4, encoding large subunits (LSU) of this enzyme heterotetrameric complex. Subcellular localization studies using green fluorescent protein (GFP) fusion constructs indicate that OsAGPS2a, the product of the leaf-preferential transcript of OsAGPS2, and OsAGPS1, OsAGPL1, OsAGPL3, and OsAGPL4 are plastid-targeted isoforms. In contrast, two isoforms, SSU OsAGPS2b which is a product of a seed-specific transcript of OsAGPS2, and LSU OsAGPL2, are localized in the cytosol. Analysis of osagps2 and osagpl2 mutants revealed that a lesion of one of the two cytosolic isoforms, OsAGPL2 and OsAGPS2b, causes a shrunken endosperm due to a remarkable reduction in starch synthesis. In leaves, however, only the osagps2 mutant appears to severely reduce the transitory starch content. Interestingly, the osagps2 mutant was indistinguishable from wild type during vegetative plant growth. Western blot analysis of the osagp mutants and wild type plants demonstrated that OsAGPS2a is an SSU isoform mainly present in leaves, and that OsAGPS2b and OsAGPL2 are the major SSU and LSU isoforms, respectively, in the endosperm. Finally, we propose a spatiotemporal complex model of OsAGP SSU and LSU isoforms in leaves and in developing endosperm of rice plants.

An Escherichia coli mutant producing a truncated inactive form of GlgC synthesizes glycogen: further evidences for the occurrence of various important sources of ADPglucose in enterobacteria

AC70R1-504 Escherichia coli mutants possess a glgC* gene with a nucleotide change resulting in a premature stop codon that renders a truncated, inactive form of GlgC. Cells over-expressing the wild type glgC, but not those over-expressing the AC70R1-504 glgC*, accumulated high ADPglucose and glycogen levels. AC70R1-504 mutants accumulated glycogen, whereas DeltaglgCAP deletion mutants lacking the whole glycogen biosynthetic machinery displayed a glycogen-less phenotype. AC70R1-504 cells with enhanced glycogen synthase activity accumulated high glycogen levels. By contrast, AC70R1-504 cells with high ADPG hydrolase activity accumulated low glycogen. These data further confirm that enterobacteria possess various sources of ADPglucose linked to glycogen biosynthesis.

A Dielectric Relaxation Study of Lithium Salt Solutions in Dimethylsulfoxide

The dielectric spectra of LiCl, LiClO

Expression of barley ADP-glucose pyrophosphorylase in Escherichia coli: processing and regulatory considerations

Full length cDNAs for barley ADP-glucose pyrophosphorylase (AGPase) coding for the large subunits of the endosperm and leaf homologues of the enzyme (AGPase-S1 and -S2, respectively) and for the small subunit protein from endosperm (AGPase-B1), have been expressed in Escherichia coli. The cDNAs for AGPase-S1 and -S2 required different induction conditions for their maximal expression and they encoded immunologically distinct proteins. The AGPase-S1 that was produced by E. coli had the same M(r) (58 kDa) as the corresponding protein in barley crude endosperm extracts, whereas the bacteria-produced AGPase-S2 (55 kDa) was larger than its counterpart from barley leaf preparations (53 kDa). An enzymatically active AGPase expressed in E. coli from a double construct containing cDNAs for AGPase-S1 and -B1 subunits was insensitive to the activation by 3-phosphoglycerate and to inhibition by inorganic phosphate, similarly to the enzyme in barley endosperm. Neither AGPase-S1 nor -B1 were active when expressed alone in the bacteria. The data are discussed with respect to possible mechanisms of intracellular targeting of immature AGPase-S proteins in barley tissues and regarding previous data on effector regulation of the barley enzyme.