18. asırda "Hırkai şerif" ziyareti

Taha Toros Arşivi, Dosya No: 120-Saraylar. Not: Gazetenin "Tarihten Sahifeler" köşesinde yayımlanmıştır.İstanbul Kalkınma Ajansı (TR10/14/YEN/0033) İstanbul Development Agency (TR10/14/YEN/0033

DNA repair mechanisms in UV-B tolerant plants

Understanding the mechanisms responsible for UV-B irradiation-induced DNA damage repair in plants is important for agricultural technology in that it will potentially enable the development of plants with enhanced growth rates and crop yields. Unlike yeast and mammalian cells, intact plants use sunlight for photosynthesis and are thus chronically exposed to the UV band wavelengths present in solar radiation. UV induces DNA damage, which can be corrected by DNA repair mechanisms such as photoreactivation and excision repair. Recently, details of several DNA repair mechanisms have become clear in plants. We made transgenic rice overexpressing genes involved in excision repair or plant-specific DNA repair, and measured their tolerance to UV-B. We found that OsUV-DDB2 and OsSEND-1 transgenic lines had higher tolerance to UV-B than the wild type. In this review, recent advances in understanding repair of DNA damage from UV-B radiation in plants and the prospects for the development of UV resistant plants are discussed

A plant homologue of 36 kDa subunit of replication factor C: Molecular cloning and characterization

We have isolated a 1.47 kb cDNA from rice (Oryza sativa) encoding plant homologue of a small subunit of Replication Factor C (RFC), and designated OsRFC3. The open reading frame of OsRFC3 encoded a predicted product of 367 amino acid residues with a molecular mass of 40.3 kDa. Genomic analysis indicated that the subunit existed as a single copy per genome. The transcript was highly expressed in root tips that contain root apical meristem, and the young and mature leaves. The mRNA level of OsRFC3 was significantly reduced when the cell proliferation was temporarily halted by removal of sucrose from the growth medium. When the growth-halted cells began to re-grow following addition of sucrose to the medium, OsRFC3 was again expressed at high level. The roles of OsRFC3 in plant DNA replication is discussed. © 2001 Elsevier Science Ireland Ltd

Plant DNA polymerase λ, a DNA repair enzyme that functions in plant meristematic and meiotic tissues

Little is known about the functions of DNA polymerase λ (Pol λ) recently identified in mammals. From the genomic sequence information of rice and Arabidopsis, we found that Pol λ may be the only member of the X-family in higher plants. We have succeeded in isolating the cDNA and recombinant protein of Pol λ in a higher plant, rice (Oryza sativa L. cv. Nipponbare) (OsPol λ). OsPol λ had activities of DNA polymerase, terminal deoxyribonucleotidyl transferase and deoxyribose phosphate lyase, a marker enzyme for base excision repair. It also interacted with rice proliferating cell nuclear antigen (OsPCNA) in a pull-down assay. OsPCNA increased the processivity of OsPol λ. Northern blot analysis showed that the level of OsPol λ expression correlated with cell proliferation in meristematic and meiotic tissues, and was induced by DNA-damaging treatments. These properties suggest that plant Pol λ is a DNA repair enzyme which functions in plant meristematic and meiotic tissues, and that it can substitute for Pol β and terminal deoxyribonucleotidyl transferase

Characterization of T-DNA insertion mutants and RNAi silenced plants of Arabidopsis thaliana UV-damaged DNA binding protein 2 (AtUV-DDB2)

The human UV-damaged DNA binding protein (UV-DDB), a heterodimeric protein composed of 127 kDa (UV-DDB1) and 48 kDa (UV-DDB2) subunits, has been shown to be involved in DNA repair. To elucidate the in vivo function of plant UV-DDB2, we have analyzed T-DNA insertion mutants of the Arabidopsis thaliana UV-DDB2 subunit (atuv-ddb2 mutants) and AtUV-DDB2 RNAi silenced plants (atuv-ddb2 silenced plants). atuv-ddb2 mutants and atuv-ddb2 silenced plants were both viable, suggesting that AtUV-DDB2 is not essential for survival. Interestingly, both plant types showed a dwarf phenotype, implying impaired growth of the meristem. To the best of our knowledge, this is the first occasion that a dwarf phenotype has been found to be associated with a UV-DDB2 mutation in either plants or animals. The mutants also demonstrated increased sensitivity to UV irradiation, methyl methanesulfonate and hydrogen peroxide treatment, indicating that AtUV-DDB2 is also involved in DNA repair. Our results lead us to suggest that not only does AtUV-DDB2 function in DNA repair, it also has a direct or indirect influence on cell proliferation in the plant meristem. © Springer 2006

Plant-specific regulation of replication protein A2 (OsRPA2) from rice during the cell cycle and in response to ultraviolet light exposure

DNA replication is a process that is highly conserved among eukaryotes. Nonetheless, little is known about the proteins involved in it in plants. Replication protein A (RPA) is a heterotrimeric, single-stranded DNA-binding protein with several functions in DNA metabolism in humans and yeast and supposedly also in plants. Here we report on the regulation of OsRPA2, the 32-kDa subunit of RPA from rice (Oryza sativa L.). We found conserved regulation mechanisms at the level of gene expression between animal and plant RPA2 genes and distinct features of OsRPA2 regulation at the level of protein expression. Unlike in animals or in yeast, protein abundance in rice was regulated in a cell cycle phase-specific manner and was altered after UV-C light exposure. On the other hand, posttranslational modification through phosphorylation did not appear to play a pivotal role in rice as it does in animal cells. Our results indicate that plant-specific mechanisms of regulation have evolved for RPA2 within the generally well-conserved process of DNA replication, suggesting specific requirements for regulation of DNA metabolism in plants as compared to other eukaryotes

Molecular cloning and characterization of a plant homologue of the origin recognition complex 1 (ORC1)

By using the rice EST database, we have isolated a 2.8 kb cDNA, termed Oryza sativa ORC1 (OsORC1), from rice (O. sativa) encoding a protein that shows homology with the eukaryotic ORC1 proteins. Alignment of the OsORC1 protein sequence with the sequence of ORC1 from human and yeasts S. cerevisiae and S. pombe showed a high degree of sequence homology (38.7, 32.9 and 35.0% identity, respectively), particularly around the C-terminal region containing the CDC-NTP domain. Interestingly, the OsORC1 protein had an A + T hook-like motif, which was not present in the human or yeast genes. Genomic analysis indicated that OsORC1 existed as a single copy per genome. OsORC1 transcripts were expressed strongly in root tips and weakly in young leaves containing root apical meristem and marginal meristem, respectively. No expression was detected in the mature leaves. The level of OsORC1 expression was significantly reduced when cell proliferation was temporarily halted by the removal of sucrose from the growth medium. When the growth-halted cells began to re-grow following addition of sucrose to the medium, OsORC1 was again expressed at high levels. These results suggested that OsORC1 is required for cell proliferation. The role of OsORC1 in plant DNA replication will be discussed. (C) 2000 Elsevier Science Ireland Ltd

Two types of replication protein A 70 kDa subunit in rice, Oryza sativa: Molecular cloning, characterization, and cellular & tissue distribution

Replication protein A (RPA), which is comprised of three subunits, is an important factor involved in DNA replication, repair, and transcription. We isolated and characterized 70 and 32 kDa subunits of RPA from rice (Oryza sativa cv. Nipponbare) termed OsRPA70a and OsRPA32. OsRPA70a shows a low level of homology with OsRPA1 which was isolated from deepwater rice (Oryza sativa cv. Pin Gaew 56), previously. We also succeeded to isolate OsRPA70b which is homologue to OsRPA1 from Oryza sativa cv. Nipponbare. OsRPA70a shows only 33.8% sequence identity with OsRPA70b, indicating that two different types of 70 kDa RPA subunits are present in Oryza sativa cv. Nipponbare. These subunits showed differences in their expression patterns among tissues. The transcripts of OsRPA70a and OsRPA32 were expressed strongly in proliferating tissues such as root tips and young leaves that contain root apical meristem and marginal meristem, respectively, and weakly in the mature leaves which have no proliferating tissues. On the other hand, OsRPA70b was expressed mostly in the proliferating tissues. The roles of these molecules in plant DNA replication and DNA repair are discussed. © 2001 Elsevier Science B.V. All rigths reserved

Characterization of Rad6 from a higher plant, rice (Oryza sativa L.) and its interaction with Sgt1, a subunit of the SCF ubiquitin ligase complex

We report here the existence of interactions between a ubiquitin- conjugating enzyme, Rad6, from rice, Oryza sativa L. cv. Nipponbare (OsRad6), and Sgt1 (OsSgt1), a novel subunit of the SCF ubiquitin ligase complex. Rad6 is not only related to post-replicational repair but also to the proteasome system, while Sgt1 has a function in kinetochore assembly. The relationship between the two is unexpected, but of great interest. The open reading frames of OsRad6 and OsSgt1 encode predicted products of 152 and 367 amino acid residues, respectively, with molecular weights of 17.3 and 40.9kDa. Two-hybrid and pull-down analyses indicated that OsRad6 binds to OsSgt1, and transcripts of both OsRad6 and OsSgt1 were found to be strongly expressed only in the proliferating tissues such as the shoot apical meristem, suggesting that their expression is cell cycle-dependent. The amount of the Rad6 mRNA in cultured cells increased rapidly after division was halted, and mRNA levels of Rad6 and Sgt1 were induced by UV- and DNA-damaging agents such as MMS or H 2O2. The Rad6 pathway for repair or the proteasome system may thus require Sgt1 as ubiquitin-conjugating enzyme. © 2003 Elsevier Inc. All rights reserved