Degradation of the D1 protein of photosystem II under illumination in vivo: two different pathways involving cleavage or intermolecular cross-linking

The D1 protein of the photosystem II reaction center turns over the most rapidly of all the proteins of the thylakoid membrane under illumination in vivo. In vitro, the D1 protein sustained cleavage in a surface-exposed loop (DE loop) or cross-linking with another reaction center protein, the D2 protein or cytochrome b(559), under illumination. We found that the D1 protein was damaged in essentially the same way in vivo, although the resultant fragments and cross-linked adducts barely accumulated due to digestion by proteases. In vitro studies detected a novel stromal protease(s) that digested the adducts but not the monomeric D1 protein. These observations suggest that, in addition to cleavage, the cross-linking reactions themselves are processes involved in complete degradation of the D1 protein in vivo. Peptide mapping experiments located the cross-linking sites with the D2 protein among residues 226-244, which includes the cross-linking site with cytochrome b(559) [Barbato, R., et al. (1995) J. Biol. Chem. 270, 24032-24037], in the N-terminal part of the DE loop, while N-terminal amino acid sequencing of the fragment located the cleavage site around residue 260 in the C-terminal part of the loop. We propose a model explaining the occurrence of simultaneous cleavage and cross-linking and discuss the mechanisms of complete degradation of the D1 protein in vivo.     

The rice retrotransposon Tos17 prefers low-copy-number sequences as integration targets

The rice retrotransposon Tos17 is highly activated by tissue culture. To evaluate the impact of transposition of Tos17 on the rice genome and examine its utility for insertional mutagenesis, more than 100 sequences flanking newly transposed Tos17 copies were characterised. The 5-bp target-site duplications flanking Tos17 did not show any consensus sequence, and preferred nucleotides, A/T and G/C, were only found at the second and third nucleotides from both ends of the target site duplications, respectively, indicating that Tos17 has relatively low target-site specificity at the nucleotide sequence level. Integration targets were widely distributed over the chromosomes; however, preferential integration into the sucrose synthase 2 gene and into Tos17 itself was demonstrated by PCR screening using pooled DNA prepared from the mutant population. Hybridisation studies indicated that Tos17 preferentially integrates into low-copy-number regions of the genome. In agreement with this result, about 30% of flanking sequences examined showed significant homology to known genes. Taken together, these results show that Tos17 can have a significant impact on the rice genome and can be used as a tool for efficient insertional mutagenesis.     

Target site specificity of the Tos17 retrotransposon shows a preference for insertion within genes and against insertion in retrotransposon-rich regions of the genome

Because retrotransposons are the major component of plant genomes, analysis of the target site selection of retrotransposons is important for understanding the structure and evolution of plant genomes. Here, we examined the target site specificity of the rice retrotransposon Tos17, which can be activated by tissue culture. We have produced 47,196 Tos17-induced insertion mutants of rice. This mutant population carries approximately 500,000 insertions. We analyzed >42,000 flanking sequences of newly transposed Tos17 copies from 4316 mutant lines. More than 20,000 unique loci were assigned on the rice genomic sequence. Analysis of these sequences showed that insertion events are three times more frequent in genic regions than in intergenic regions. Consistent with this result, Tos17 was shown to prefer gene-dense regions over centromeric heterochromatin regions. Analysis of insertion target sequences revealed a palindromic consensus sequence, ANGTT-TSD-AACNT, flanking the 5-bp target site duplication. Although insertion targets are distributed throughout the chromosomes, they tend to cluster, and 76% of the clusters are located in genic regions. The mechanisms of target site selection by Tos17, the utility of the mutant lines, and the knockout gene database are discussed. --The nucleotide sequence data were uploaded to the DDBJ, EMBL, and GenBank nucleotide sequence databases under accession numbers AG020727 to AG025611 and AG205093 to AG215049.     

Pdk1 kinase regulates basal disease resistance through the OsOxi1-OsPti1a phosphorylation cascade in rice

The AGC kinase OsOxi1, which has been isolated as an interactor with OsPti1a, positively regulates basal disease resistance in rice. In eukaryotes, AGC kinase family proteins are regulated by 3-phosphoinositide-dependent protein kinase 1 (Pdk1). In Arabidopsis, AtPdk1 directly interacts with phosphatidic acid, which functions as a second messenger in both biotic and abiotic stress responses. However, the functions of Pdk1 are poorly understood in plants. We show here that OsPdk1 acts upstream of the OsOxi1-OsPti1a signal cascade in disease resistance in rice. OsPdk1 interacts with OsOxi1 and phosphorylates the Ser283 residue of OsOxi1 in vitro. To investigate whether OsPdk1 is involved in immunity that is triggered by microbial-associated molecular patterns, we analyzed the phosphorylation status of OsPdk1 in response to chitin elicitor. Like OsOxi1, OsPdk1 is rapidly phosphorylated in response to chitin elicitor, suggesting that OsPdk1 participates in signal transduction through pathogen recognition. The overexpression of OsPdk1 enhanced basal resistance against a blast fungus, Magnaporthe oryzae, and a bacterial pathogen, Xanthomonas oryzae pv. oryzae (Xoo). Taken together, these results suggest that OsPdk1 positively regulates basal disease resistance through the OsOxi1-OsPti1a phosphorylation cascade in rice.     

Unique features of the rice blast resistance <Emphasis Type="Italic">Pish</Emphasis> locus revealed by large scale retrotransposon-tagging

Background  

R gene-mediated resistance is one of the most effective mechanisms of immunity against pathogens in plants. To date some components that regulate the primary steps of plant immunity have been isolated, however, the molecular dissection of defense signaling downstream of the R proteins remains to be completed. In addition, R genes are known to be highly variable, however, the molecular mechanisms responsible for this variability remain obscure.     

A novel RNA-recognition-motif protein is required for premeiotic G1/S-phase transition in rice (Oryza sativa L.)

The molecular mechanism for meiotic entry remains largely elusive in flowering plants. Only Arabidopsis SWI1/DYAD and maize AM1, both of which are the coiled-coil protein, are known to be required for the initiation of plant meiosis. The mechanism underlying the synchrony of male meiosis, characteristic to flowering plants, has also been unclear in the plant kingdom. In other eukaryotes, RNA-recognition-motif (RRM) proteins are known to play essential roles in germ-cell development and meiosis progression. Rice MEL2 protein discovered in this study shows partial similarity with human proline-rich RRM protein, deleted in Azoospermia-Associated Protein1 (DAZAP1), though MEL2 also possesses ankyrin repeats and a RING finger motif. Expression analyses of several cell-cycle markers revealed that, in mel2 mutant anthers, most germ cells failed to enter premeiotic S-phase and meiosis, and a part escaped from the defect and underwent meiosis with a significant delay or continued mitotic cycles. Immunofluorescent detection revealed that T7 peptide-tagged MEL2 localized at cytoplasmic perinuclear region of germ cells during premeiotic interphase in transgenic rice plants. This study is the first report of the plant RRM protein, which is required for regulating the premeiotic G1/S-phase transition of male and female germ cells and also establishing synchrony of male meiosis. This study will contribute to elucidation of similarities and diversities in reproduction system between plants and other species.     

Inhibition of c-Jun NH2-terminal kinase activity improves ischemia/reperfusion injury in rat lungs

Although c-Jun NH(2)-terminal kinase (JNK) has been implicated in the pathogenesis of transplantation-induced ischemia/reperfusion (I/R) injury in various organs, its significance in lung transplantation has not been conclusively elucidated. We therefore attempted to measure the transitional changes in JNK and AP-1 activities in I/R-injured lungs. Subsequently, we assessed the effects of JNK inhibition by the three agents including SP600125 on the degree of lung injury assessed by means of various biological markers in bronchoalveolar lavage fluid and histological examination including detection of apoptosis. In addition, we evaluated the changes in p38, extracellular signal-regulated kinase, and NF-kappaB-DNA binding activity. I/R injury was established in the isolated rat lung preserved in modified Euro-Collins solution at 4 degrees C for 4 h followed by reperfusion at 37 degrees C for 3 h. We found that AP-1 was transiently activated during ischemia but showed sustained activation during reperfusion, leading to significant lung injury and apoptosis. The change in AP-1 was generally in parallel with that of JNK, which was activated in epithelial cells (bronchial and alveolar), alveolar macrophages, and smooth muscle cells (bronchial and vascular) on immunohistochemical examination. The change in NF-kappaB qualitatively differed from that of AP-1. Protein leakage, release of lactate dehydrogenase and TNF-alpha into bronchoalveolar lavage fluid, and lung injury were improved, and apoptosis was suppressed by JNK inhibition. In conclusion, JNK plays a pivotal role in mediating lung injury caused by I/R. Therefore, inhibition of JNK activity has potential as an effective therapeutic strategy for preventing I/R injury during lung transplantation.