High-salinity, drought, and low temperature are three common environmental stress factors that seriously influence plant growth and development worldwide. Recently, microRNAs (miRNAs) have emerged as a class of gene expression regulators that have also been linked to stress responses. However, the relationship between miRNA expression and stress responses is just beginning to be explored. Here, we identified 14 stress-inducible miRNAs using microarray data in which the effects of three abiotic stresses were surveyed in Arabidopsis thaliana. Among them, 10 high-salinity-, four drought-, and 10 cold-regulated miRNAs were detected, respectively. miR168, miR171, and miR396 responded to all of the stresses. Expression profiling by RT-PCR analysis showed great cross-talk among the high-salinity, drought, and cold stress signaling pathways. The existence of stress-related elements in miRNA promoter regions provided further evidence supporting our results. These findings extend the current view about miRNA as ubiquitous regulators under stress conditions.

Ethylene responsive factors (ERFs) are important plant-specific transcription factors, some of which have been demonstrated to interact with the ethylene-responsive GCC box and the dehydration-responsive element (DRE); however, data on the roles of ERF proteins in connection with various signaling pathways are limited. In this research, we used the GCC box, an essential cis-acting element responsive to ethylene and methyl jasmonate (MeJA), as bait in a yeast one-hybrid system to isolate transcription factors from tomato (Lycopersicon esculentum Mill.). One of the cDNAs, which was designated Jasmonate and Ethylene Response Factor 1 (JERF1), encodes an ERF protein, containing a conserved ERF DNA-binding motif and functioning as a transcriptional activator in yeast through targeting to the nucleus in onion (Allium cepa L.) epidermal cells. Biochemical analysis revealed that JERF1 bound not only to the GCC box but also to the DRE sequence. Expression of the JERF1 gene in tomato was induced by ethylene, MeJA, abscisic acid (ABA) and salt treatment, indicating that JERF1 might act as a connector among different signal transduction pathways. Further research with transgenic JERF1 tobacco (Nicotiana tabacum L.) plants indicated that overexpressing JERF1 activated expression of GCC box-containing genes such as osmotin, GLA, Prb-1b and CHN50 under normal growth conditions, and subsequently resulted in enhanced tolerance to salt stress, suggesting that JERF1 modulates osmotic tolerance by activation of downstream gene expression through interaction with the GCC box or DRE.

Ribonucleotide reductases catalyze the formation of deoxyribonucleotides by the reduction of the corresponding ribonucleotides. Eukaryotic ribonucleotide reductases are ␣ 2 ␤ 2 tetramers; each of the larger, ␣ subunits possesses binding sites for substrate and allosteric effectors, and each of the smaller, ␤ subunits contains a binuclear iron complex. The iron complex interacts with a specific tyrosine residue to form a tyrosyl free radical which is essential for activity. Previous work has identified two genes in the yeast Saccharomyces cerevisiae, RNR1 and RNR3, that encode ␣ subunits and one gene, RNR2, that encodes a ␤ subunit. Here we report the identification of a second gene from this yeast, RNR4, that encodes a protein with significant similarity to the ␤-subunit proteins. The phenotype of rnr4 mutants is consistent with that expected for a defect in ribonucleotide reductase; rnr4 mutants are supersensitive to the ribonucleotide reductase inhibitor hydroxyurea and display an S-phase arrest at their restrictive temperature. rnr4 mutant extracts are deficient in ribonucleotide reductase activity, and this deficiency can be remedied by the addition of exogenous Rnr4p. As is the case for the other RNR genes, RNR4 is induced by agents that damage DNA. However, Rnr4p lacks a number of sequence elements thought to be essential for iron binding, and mutation of the critical tyrosine residue does not affect Rnr4p function. These results suggest that Rnr4p is catalytically inactive but, nonetheless, does play a role in the ribonucleotide reductase complex.Ribonucleotide reductases catalyze the formation of deoxyribonucleotides by the reduction of the corresponding ribonucleotides. Three classes of ribonucleotide reductases have been well characterized (24). Class I enzymes are found in all eukaryotes and some prokaryotes. The best-studied class I enzyme is the Escherichia coli ribonucleotide reductase (10, 30), an ␣ 2 ␤ 2 tetramer that can be decomposed to two catalytically inactive homodimers, R1 (␣ 2 ) and R2 (␤ 2 ). Each of the larger ␣ subunits possesses binding sites for substrate and allosteric effectors and also contains several redox-active cysteine residues. Each of the smaller ␤ subunits contains a binuclear Fe(III) complex. The X-ray structure of E. coli R2 reveals that the iron ions are bridged by both an O 2Ϫ ion and the carboxyl group of a glutamate residue (22). Each iron is further liganded by two carboxyl oxygen atoms from aspartate or glutamate residues, a histidine N␦ residue, and a water molecule. The recently solved structure of the mouse R2 protein indicates that the iron-binding center of eukaryotic proteins is similar to that of the E. coli protein (17). The iron complex interacts with a specific tyrosine residue to form a tyrosyl free radical which is essential for activity. The enzyme is inhibited by hydroxyurea, which specifically quenches the tyrosyl radical (19).Amino acid sequence alignments of the class 1 R2 proteins from different species identify 16 residues that are conserved in a...

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