Transcript regulation in response to high salinity was investigated for salt-tolerant rice (var Pokkali) with microarrays including 1728 cDNAs from libraries of salt-stressed roots. NaCl at 150 mM reduced photosynthesis to one tenth of the prestress value within minutes. Hybridizations of RNA to microarray slides probed for changes in transcripts from 15 min to 1 week after salt shock. Beginning 15 min after the shock, Pokkali showed upregulation of transcripts. Approximately 10% of the transcripts in Pokkali were significantly upregulated or downregulated within 1 hr of salt stress. The initial differences between control and stressed plants continued for hours but became less pronounced as the plants adapted over time. The interpretation of an adaptive process was supported by the similar analysis of salinity-sensitive rice (var IR29), in which the immediate response exhibited by Pokkali was delayed and later resulted in downregulation of transcription and death. The upregulated functions observed with Pokkali at different time points during stress adaptation changed over time. Increased protein synthesis and protein turnover were observed at early time points, followed by the induction of known stress-responsive transcripts within hours, and the induction of transcripts for defenserelated functions later. After 1 week, the nature of upregulated transcripts (e.g., aquaporins) indicated recovery.

Drought‐affected plants experience more than just desiccation of their organs due to water deficit. Plants transpire 1000 times more molecules of water than of CO₂ fixed by photosynthesis in full sunlight. One effect of transpiration is to cool the leaves. Accordingly, drought brings about such multi‐stresses as high temperatures, excess photoradiation and other factors that affect plant viability. Wild watermelon serves as a suitable model system to study drought responses of C₃ plants, since this plant survives drought by maintaining its water content without any wilting of leaves or desiccation even under severe drought conditions. Under drought conditions in the presence of strong light, wild watermelon accumulates high concentrations of citrulline, glutamate and arginine in its leaves. The accumulation of citrulline and arginine may be related to the induction of DRIP‐1, a homologue of ArgE in Escherichia coli, where it functions to incorporate the carbon skeleton of glutamate into the urea cycle. Immunogold electron microscopy reveals the enzyme to be confined exclusively to the cytosol. DRIP‐1 is also induced by treating wild watermelon with 150 mm NaCl, but is not induced following treatment with 100 µm abscisic acid. The salt treatment causes the accumulation of γ‐aminobutyrate, glutamine and alanine, in addition to a smaller amount of citrulline. Citrulline may function as a potent hydroxyl radical scavenger.

Summary: During early chloroplast differentiation, the regulation of the plastid genetic system including transcription and translation differs greatly from that in the mature chloroplast, suggesting the existence of a stage‐dependent mechanism that regulates the chloroplast genetic system during this period. The virescent‐1 (v₁) mutant of rice (Oryza sativa) is temperature‐conditional and develops chlorotic leaves under low‐temperature conditions. We reported previously that leaf chlorosis in the v₁ mutant is caused by blockage of the activation of the chloroplast genetic system during early leaf development. Here we identify the V₁ gene, which encodes a chloroplast‐localized protein NUS1. Accumulation of NUS1 specifically occurred in the pre‐emerged immature leaves, and is enhanced by low‐temperature treatment. The C‐terminus of NUS1 shows structural similarity to the bacterial antitermination factor NusB, which is known to play roles in the regulation of ribosomal RNA transcription. The RNA‐immunoprecipitation and gel mobility shift assays indicated that NUS1 binds to several regions of chloroplast RNA including the upstream leader region of the 16S rRNA precursor. In the leaves of the NUS1‐deficient mutant, accumulation of chloroplast rRNA during early leaf development was impaired and chloroplast translation/transcription capacity was severely suppressed under low temperature. Our results suggest that NUS1 is involved in the regulation of chloroplast RNA metabolism and promotes the establishment of the plastid genetic system during early chloroplast development under cold stress conditions.

Clostridium acetobutylicum and Clostridium aminovalericum , both obligatory anaerobes, grow normally after growth conditions are changed from anoxic to microoxic, where the cells consume oxygen proficiently. In C. aminovalericum , a gene encoding a previously characterized H ₂ O-forming NADH oxidase, designated noxA , was cloned and sequenced. The expression of noxA was strongly upregulated within 10 min after the growth conditions were altered to a microoxic state, indicating that C. aminovalericum NoxA is involved in oxygen metabolism. In C. acetobutylicum , genes suggested to be involved in oxygen metabolism and genes for reactive oxygen species (ROS) scavenging were chosen from the genome database. Although no clear orthologue of C. aminovalericum NoxA was found, Northern blot analysis identified many O ₂ -responsive genes (e.g., a gene cluster [ CAC2448 to CAC2452 ] encoding an NADH rubredoxin oxidoreductase-A-type flavoprotein-desulfoferrodoxin homologue-MerR family-like protein-flavodoxin, an operon [ CAC1547 to CAC1549 ] encoding a thioredoxin-thioredoxin reductase-glutathione peroxidase-like protein, an operon [ CAC1570 and CAC1571 ] encoding two glutathione peroxidase-like proteins, and genes encoding thiol peroxidase, bacterioferritin comigratory proteins, and superoxide dismutase) whose expression was quickly and synchronously upregulated within 10 min after flushing with 5% O _2. The corresponding enzyme activities, such as NAD(P)H-dependent peroxide (H ₂ O ₂ and alkyl hydroperoxides) reductase, were highly induced, indicating that microoxic growth of C. acetobutylicum is associated with the expression of a number of genes for oxygen metabolism and ROS scavenging.

The primary structure of an nir gene cluster necessary for production of active dissimilatory nitrite reductase was determined from Pseudomonas aeruginosa. Seven open reading frames, designated nirDLGHJEN, were identified downstream of the previously reported nirSMCF genes. From nirS through nirN, the stop codon of one gene and the start codon of the next gene were closely linked, suggesting that nirSMCFDLGHJEN are expressed from a promoter which regulates the transcription of nirSM. The amino acid sequences deduced from the nirDLGH genes were homologous to each other. A gene, designated nirJ, which encodes a protein of 387 amino acids, showed partial identity with each of the nirDLGH genes. The nirE gene encodes a protein of 279 amino acids homologous to S-adenosyl-L-methionine:uroporphyrinogen III methyltransferase from other bacterial strains. In addition, NirE shows 21.0% identity with NirF in the N-terminal 100-amino-acid residues. A gene, designated nirN, encodes a protein of 493 amino acids with a conserved binding motif for heme c (CXXCH) and a typical N-terminal signal sequence for membrane translocation. The derived NirN protein shows 23.9% identity with nitrite reductase (NirS). Insertional mutation and complementation analyses showed that all of the nirFDLGHJE genes were necessary for the biosynthesis of heme d1.