It is worth noting that in S cerevisiae

It is worth noting that in S. cerevisiae, the control of two cell-cycle checkpoints requiring Mcm1 involves two very different mechanisms. At the G1/S transition, Mcm1 transactivation activity is inhibited by Yox1 and Yhp1 repressors, whose expression is periodically regulated through the cell cycle. In contrast, the G2/M transition requires the activation of the Mcm1–Fkh2 complex by a phosphorylation event and the recruitment of the Ndd1 factor. Interestingly, Mcm1 associated with its arginine cofactors Arg80 and Arg81 can act either as a repressor of arginine anabolic genes or as an activator of arginine catabolic genes. How this is achieved is still unknown, but a model similar to the one presented for flower development in Fig. 8 might be applicable for the control of arginine coregulated genes. The “arginine boxes” are rather large DNA sequences (from 30 to 60 nucleotides), comprising two degenerated Mcm1 9090 separated by a GC-rich region. It was hypothesized that the two MADS box proteins Arg80 and Mcm1 could interact, potentially as heterodimers, with the P-like sites and induce DNA bending, thus allowing Arg81, the arginine sensor, to interact with the GC-rich region leading to the formation of a stable protein–DNA complex (Messenguy and Dubois, 2000). Differences in promoter architecture or in chromatin structure could then account for repression or activation of gene expression by the same subset of interacting proteins. Supporting this hypothesis is the recent observation that the repression of the anabolic gene ARG1 by arginine requires the integrity of the SAGA histone acetyltransferase complex and of the E2 ubiquitin conjugase Rad6 Ricci et al., 2002, Turner et al., 2002.
Acknowledgements
Introduction Aerobic organisms are continuously exposed to oxygen which renders them prone to damage generated by oxygen-derived free radicals. Oxidative stress is mediated by the reactive oxygen species (ROS), including the superoxide anion (O2−), hydrogen peroxide (H2O2), and the hydroxyl radical (OH) and reactive nitrogen species (RNS) which include the free radical nitric oxide (NO) and peroxynitrite (ONOO), a product of the reaction between NO and O2[1], [2]. Generation of ROS in non-stressed cells is principally restricted to organelles such as chloroplasts, mitochondria and peroxisomes in which they are produced as by-products of electron chains and metabolic reactions. ROS are also produced by NADPH oxidases, peroxidases and amine oxidases at the plasma membrane, cell wall or the apoplast as well as by FAD-containing enzymes of the oxidative protein folding machinery in the secretory pathway [3], [4]. Exposure to various environmental factors is also leading to oxidative stress which can cause oxidative damage to proteins, DNA, and lipids [2]. To scavenge excessive ROS/RNS, plants exhibit a large battery of enzymes like peroxidases, catalases, superoxide dismutases and of antioxidant molecules like ascorbate, glutathione, tocopherol, carotenoides. ROS and RNS act also as signaling molecules in important physiological processes. In plants, they are involved in developmental programs like root development, stomatal closure or programmed cell death. They are also key signaling molecules for the responses to abiotic and biotic stresses, activating transcriptional, post-transcriptional and post-translational responses [5], [6]. The nuclear compartment that orchestrates genetic programs of cell life is particularly sensitive to the deleterious effects of oxidation. ROS, and particularly H2O2, probably diffuse through membranes and invade neighboring compartments, including the nucleus [6]. Experiments performed with isolated tobacco BY-2 nuclei also suggest that plant nuclei are an active source of production of ROS, in particular of H2O2[7]. Increasing evidence has highlighted the role of redox signaling in the regulation of many nuclear proteins like transcription factors, kinases and chromatin-modifying enzymes [8]. The redox regulation generally occurs through modification of the redox state of thiol residues. Solvant exposed thiols are prone to oxidation especially in a basic environment since thiol deprotonation leads to formation of a thiolate residue (R-S-), a nucleophilic residue sensitive to oxidation. In the presence of ROS, such as H2O2, oxidation leads to successive oxidation to sulfenic (R-SOH), sulfinic (R-SO2H) and sulfonic (R-SO2H) acids [9]. Thiol groups can also be oxidized by reactive nitrogen species (RNS) or oxidized glutathione (GSSG) resulting in S-nitrosylation (R-SNO) or S-glutathionylation (R-S-SG) [10], [11], [12]. Two closely situated cysteine residues can also get oxidized to form a disulfide bridge (SS). All these modifications can alter the structure and/or the activity of many proteins in all the cellular compartments, including the nucleus.