The alterations in structure and function of photosystem II (PS II) during the senescence of primary leaves of wheat seedlings have been compared with the changes induced by ultraviolet-B (UV-B) radiation in the presence or absence of photosynthetically active radiation (PAR). The results indicated that the senescence-induced loss in pigment content, thylakoid membrane integrity and carotenoid-to-chlorophyll (Car-to-Chl) energy transfer efficiency was intensified by exposure to UV-B radiation. Different parameters for the measurement of PS II activity, such as Chl a fluorescence, O2-evolution and thermoluminescence intensity, were altered during senescence and these alterations were furthered by UV-B irradiation. The damage of photosynthetic apparatus by UV-B exposure in the presence of PAR was less than the damage in absence of PAR. The activation of molecular defense mechanisms could be a factor in the alleviation of UV-B damage in the presence of PAR.
Plasmid pSUP5011 contains a pMBl origin of replication, an origin of transfer (oriT), and genes encoding resistance to the antibiotics ampicillin, chloramphenicol and kanamycin. pSUP5011 was conjugally mobilized from Escherichia coli into the non-heterocystous, filamentous, nitrogen-fixing cyanobacterium Plectonema boryanum UTEX 594 in the presence of a helper plasmid, RP4. Transconjugant cyanobacteria selected for resistance to kanamycin, ampicillin and chloramphenicol showed a variety of DNA rearrangements in pSUP5011. One such plasmid continued to show characteristic rearrangements following subsequent transfers into the cyanobacterium. A stable plasmid useful as a shuttle vector was isolated.
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