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The properties of Escherichia coli proteins from Arabidopsis thaliana (At), humans (Hs) and the yeast Saccharomyces cerevisiae (Sc) are analyzed and compared in order to assess the biochemical evolution of DNA polymerase [eta] (Pol[eta]) in plants. The studies have shown that the DNA sequences surrounding cyclobutane thymine dimers (T[CPD]T) has strongly affected Pol[eta] activity.

To assess how evolution might have biochemically shaped DNA polymerase 1/ (Polt/) in plants. we expressed in E.scherichia co/i proteins from .4rabidopsis thu/lana (At), humans (Hs). and the yeast Saecharomycea cereuisiae (Sc), purified them to near homogeneity, and compared their properties. Consistent with the multiple divergent amino acids within mostly conserved polymerase domains, the polymerases showed modest, appreciable, and marked differences. respectively, in salt and temperature optima for activity and thermostability. We compared abilities to extend synthetic primers past template cyclobutane thymine dimers (T[CPD]T) or undamaged T-T under physiological conditions (80-I It) mM salt). Specific activities for "standing-start" extension of synthetic primers ending opposite the second template nucleotide 3' to T-T were roughly similar. DUriTig subsequent "running-start" insertions past T-T and the next 5' (N + I) nucleotide, AtPolt7 and HsPolij appeared more processive. but DNA sequence contexts strongly affected termination probabilities. Lesion-bypass studies employed four different templates containing TICPD]Ts, and two containing pyrimidine (6--t')-pyrimidinone photoproducts (16-415). AtPol~ made the three successive insertions [opposite the TICPD]T and (N + I) nucleotidesi that define bypass nearly as well as HsPoltj and somewhat better than ScPol~. Again, sequence context elfects were profound. Interestingly, the level of insertion opposite the (N - I) nucleotide 3' to TICPD1T by HsPolt/ and especially AtPolsi, but not ScPolli. was reduced (up to 4-fold) relative to the level of insertion opposite the (N - I) nucleotide 3' to T-T. Evolutionary conservation of efficient T[CPD]T bypass by HsPolq and AtPols~ may reflect a high degree of exposure of human skin and plants to solar UV-B radiation. The depressed (N - I) insertion upstream of T[CPDIT (but not T-T) may reduce the extent of gratuitous error-prone insertion. [ABSTRACT FROM AUTHOR]

Dielectrophoretic behaviors and assembly of a binary suspension in aqueous media are examined in the presence of nonuniform alternating current (AC) electric field. A peculiar low-frequency threshold and dielectrophoresis (DEP) crossover frequency determine the applicable frequency window for binary assembly under positive DEP, which can be effectively tuned by medium conductivity and particle size, suggesting that the dynamic double-layer effect is responsible for the interfacial polarization of micrometer to submicrometer-sized particles in aqueous suspensions. Strong effects of AC-field frequency, medium conductivity, and size ratio on binary assembly morphology have been observed. A frequency-medium conductivity phase diagram is obtained to illustrate the morphological transition of assembled colloidal aggregates from segregated, ordered assemblies to inverted segregation with the appearance of amorphous phases upon increasing frequency and/or medium conductivity, which is a direct consequence of the competition between DEP and hydrodynamic mobility. Significantly, our results demonstrate a rapid method to form hybrid nanostructured materials.