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.
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 defense-related functions later. After 1 week, the nature of upregulated transcripts (e.g., aquaporins) indicated recovery.
Charge transfer in DNA is of current interest because of the involvement of charge transfer in oxidative DNA damage and electronic molecular devices. We have investigated the charge separation process via the consecutive adenine (A)-hopping mechanism using laser flash photolysis of DNA conjugated with naphthaldiimide (NDI) as an electron acceptor and phenothiazine (PTZ) as a donor. Upon the 355-nm laser flash excitation of NDI, the charge separation and recombination process between NDI and PTZ was observed. The yields of the charge separation via the consecutive A-hopping were slightly dependent upon the number of A bases between the two chromophores, while the charge recombination rate was strongly dependent upon the distance. The charge-separated state persisted over 300 micros when NDI was separated from PTZ by eight A bases. Furthermore, the rate constant of the A-hopping process was determined to be 2 x 10(10) s(-1) from an analysis of the yield of the charge separation depending on the number of A-hopping steps.
Mechanism of photo-induced electron transfer and the subsequent hole transfer in DNA has been studied extensively, but so far we are not aware of any reliable report of the observation of the long-distance hole-transfer process. In this article, we demonstrate the results of direct observation for the long-distance hole transfer in double-helical DNA over 100 Å with time-resolved transient absorption measurements. DNA conjugated with naphthalimide (NI) and phenothiazine (PTZ) (which worked as electron-acceptor and donor molecules, respectively) at both 5 ends was synthesized to observe the hole-transfer process. Site-selective charge injection into G by means of the adenine-hopping process was accomplished by excitation of NI with a 355-nm laser flash. Transient absorption around 400 nm, which was assigned to the NI radical anion, was observed immediately after the irradiation of a laser flash, indicating that the charge separation between NI and the nearest G occurred. Then, the transient absorption of the PTZ radical cation (PTZ •؉ ) at 520 nm was emerged, which was attributed to the hole transfer through DNA to the PTZ site. By monitoring the time profiles of the transient absorption of PTZ •؉ for NI-A6-(GA)n-PTZ and NI-A6-(GT)n-PTZ (n ؍ 2, 3, 4, 6, 8, 12) (base sequences correspond to those for DNA modified with NI), the long-distance hole-transfer process from G to PTZ, which occurred in the time scale of microsecond to millisecond, was observed directly. By assuming an average distance of 3.4 Å between base-pairs, total distance reaches 100 Å for n ؍ 12 sequences. Our results clearly show the direct observation of the long-distance hole transfer over 100 Å.D ouble-helical DNA, which has unique structural properties, is a promising candidate for molecular electronic devices and a scaffold for two-and three-dimensional nanostructures (1, 2). Mechanistic studies of charge transfer in DNA have attracted considerable attention because of their relevance to the development of DNA molecular wires (3) and the involvement in DNA oxidative lesion and strand cleavage (4, 5). There are many mechanistic studies for the ''one-dimensional'' conductivity in the double helix (6-8). Factors controlling charge-transfer properties in DNA (such as electronic coupling between donor and acceptor, structural dynamics, reorganization energy, etc.) have been discussed (9-14), and the mechanism for charge transfer, especially hole transfer over long distances in DNA, is the subject of experimental and theoretical studies.Generally, experimental studies of charge transfer in DNA have been performed by time-resolved measurements and electrophoresis analysis (15)(16)(17)(18)(19). A model of a multistep holehopping mechanism, which was proposed by Giese and coworkers (20), is the most widely adopted. An alternative mechanism in which delocalized hole migrates by means of a polaron-like mechanism was proposed by Schuster and coworkers (18). Strand-cleavage studies using PAGE analysis have provided valuable information about the seque...
A kinetic study of the single-step hole transfer in DNA was performed by measuring time-resolved transient absorption. DNA molecules with various sequences were designed and conjugated with naphthalimide (NI) and phenothiazine (PTZ) to investigate the sequence and distance dependence of the single-step hole transfer between guanines (Gs). Hole injection into DNA was accomplished by excitation of the NI site with a 355 nm laser pulse, and the kinetics of the hole-transfer process were investigated by monitoring the transient absorption of the PTZ radical cation (PTZ.+). Kinetic analysis of the time profile of PTZ.+ based on the kinetic model showed that the distance dependence of the hole-transfer process was significantly influenced by the DNA sequence. Results of temperature- and isotope-effect experiments demonstrated that the activation energy increased as the number of bridge bases separating the Gs increased. This is because of the distance-dependent reorganization energy and contribution of the proton-transfer process to the hole transfer in DNA.
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