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The genes rbcS and rbcL encode, respectively, the small and large subunits of the photosynthetic carbon dioxide fixation enzyme ribulose bisphosphate carboxylase͞oxygenase. There is a single rbcL gene in each chloroplast chromosome; a family of rbcS genes is located in the nuclear genome. These two genes are not ex- L eaves of C3 plants contain a single type of photosynthetic cell. Each of these cells carries on all of the reactions of oxygenic photosynthesis and contains the carbon dioxide-fixing enzyme Rubisco (ribulose-1,5-bisphosphate carboxylase͞ oxygenase). This enzyme catalyzes the fixation of CO 2 to ribulose-1,5-phosphate and the production of two molecules of 3-phosphoglycerate.Maize is a C4 plant. Its leaves have two types of major photosynthetic cells: a cylinder of bundle sheath cells (BSC) surrounds each of the vascular bundles; mesophyll cells (MC) occupy the remainder of the space between the upper epidermis and the lower epidermis. Some MC and BSC are immediately adjacent to one another. In MC, CO 2 is fixed to phosphoenolpyruvate (PEP) by PEP carboxylase to form oxaloacetate, the latter, a four-carbon acid, is reduced to malate, which is transferred to BSC. CO 2 is released from the malate in BSC. Rubisco, which refixes the CO 2 , is present in BSC but not in MC. C4 species differ with regard to the exact product of oxaloactate that is moved from MC to BSC but in all cases Rubisco is found only in one cell type.Rubisco is comprised of eight large subunits, each of about 55 kDa, and eight small subunits, each of about 13 kDa. The large subunit is the product of a single chloroplast rbcL gene per chloroplast chromosome (1, 2). The small subunits are encoded by a family of nuclear rbcS genes (3).Transcripts of rbcS and rbcL are detectable in MC and BSC in leaves of dark-grown maize seedlings. Upon illumination the transcripts increase 2-to 3-fold in abundance in BSC but become undetectable in MC (3-5). The maize nuclear gene rbcS-m3 (6) follows the general pattern of rbcS expression in maize, i.e., expression is induced in BSC and repressed in MC upon illumination of dark-grown seedlings. About 35% of the total leaf rbcS mRNA in 24 h illuminated dark-grown maize is transcribed from rbcS-m3 (3). The control of repression of this gene in MC is the subject of the present work.Using an in situ reporter gene transient expression assay, we found previously that sequences of rbcS-m3 that lie between Ϫ93 bp and ϩ64 bp of the transcription start site are required for promoting photoregulated expression in BSC (7,8). On the other hand, photoregulated partial suppression of rbcS-m3-reporter gene expression in MC was found to require gene sequences that lie between Ϫ907 and Ϫ445 bp together with sequences that lie between ϩ720 bp and ϩ957 bp. The latter are just beyond the translation stop codon within the 3Ј transcribed region of the gene, but are equally effective when relocated 5Ј to the Ϫ907 to Ϫ445 corepressor-containing region. We also found that expression of the reporter gene is suppressed in MC only ...

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Some oxovanadium compounds have shown potential to inhibit RNase activity, while at the same time not inhibiting DNase activity. Some vanadyl complexes also inhibit protein synthesis in rabbit reticulocytes, but induce activation of protein–tyrosine kinase. To gain an insight into the interaction of oxovanadium ions with proteins, the present study was designed to examine the bindings of VOSO4 and NaVO3 salts with human serum albumin (HSA) in aqueous solution at physiological pH with metal ion concentrations of 0.0001 to 1 mM and HSA (fatty acid free) concentration of 2% w/v. Gel and capillary electrophoresis (CE) and Fourier transform infrared (FT-IR) spectroscopic methods were used to determine the metal ion binding mode, association constant, and the secondary structure of the protein in the presence of the oxovanadium compounds. Gel electrophoresis results showed that a maximum of 20 vanadyl cations (VO2+) are bound per HSA molecule with strong (K1 = 7.0 × 107 M–1) and weak (K2 = 6.5 × 105 M–1) bindings. Similarly, capillary electrophoresis showed two major bindings for vanadyl cation with K1 = 1.2 × 108 M–1 and K2 = 8.5 × 105 M–1, whereas vanadate (VO–3) has only a weak binding affinity (K = 6.0 × 103 M–1) with HSA molecule. The VO–3 binds mainly to the lysine ε-amino NH+3 groups, while VO2+ binds possibly to the histidine nitrogen atom and the N-terminal of the α-amine residue. Infrared spectroscopic analysis showed metal ion binding results in major protein secondary structural changes from that of the α-helix (55.0 to 43–44%) to the β-sheet (22.0 to 23–26%), β-antiparallel (12.0 to 13–16%), and turn (11.0 to 17–18%), at high metal ion concentration. The observed spectral changes indicate a partial unfolding of the protein structure, in the presence of oxovanadium ions.Key words: oxovanadium, protein, binding mode, binding constant, secondary structure, electrophoresis, FT-IR spectroscopy.