The function of DNA during oxidative stress and its suitability as a potential building block for molecular devices depend on long-distance transfer of electrons and holes through the molecule, yet many conflicting measurements of the efficiency of this process have been reported. It is accepted that charges are transported over long distances through a multistep hopping reaction; this 'G-hopping' involves positive charges moving between guanines (Gs), the DNA bases with the lowest ionization potential. But the mechanism fails to explain the persistence of efficient charge transfer when the guanine sites are distant, where transfer rates do not, as expected, decrease rapidly with transfer distance. Here we show experimentally that the rate of charge transfer between two guanine bases decreases with increasing separation only if the guanines are separated by no more than three base pairs; if more bridging base pairs are present, the transfer rates exhibit only a weak distance dependence. We attribute this distinct change in the distance dependence of the rate of charge transfer through DNA to a shift from coherent superexchange charge transfer (tunnelling) at short distances to a process mediated by thermally induced hopping of charges between adenine bases (A-hopping) at long distances. Our results confirm theoretical predictions of this behaviour, emphasizing that seemingly contradictory observations of a strong as well as a weak influence of distance on DNA charge transfer are readily explained by a change in the transfer mechanism.

A guanine radical cation (G +• ) was site-selectively generated in double stranded DNA and the charge transfer in different oligonucleotide sequences was investigated. The method is based on the competition between a charge transfer from G +• through the DNA and its trapping reaction with H 2 O. We analyzed the hole transfer from this G +• to a GGG unit through one, two, three, and four AT base pairs and found that the rate decreases by about 1 order of magnitude with each intervening AT base pair. This strong distance dependence led to a β-value of 0.7 ( 0.1 Å -1 . Within the time scale of this assay the charge transfer nearly vanished when the G +• was separated by four AT base pairs from the GGG unit. However, if the second or the third of the four intervening AT base pairs was exchanged by a GC base pair, the rate of the hole transfer from the G +• to the GGG unit increased by 2 orders of magnitude. In addition, a long-range charge transfer over 15 base pairs could be observed in a mixed strand that contained AT as well as GC base pairs. Because G +• can oxidize G but not A bases, the long-range charge transport can be explained by a hopping of the positive charge between the intervening G bases. Thus, the overall charge transport in a mixed strand is a multistep hopping process between G bases where the individual steps contribute to the overall rate. The distance dependence is no longer described by the β value of the superexchange mechanism.

Pole development is coordinated with the Caulobacter crescentus cell cycle by two-component signaling proteins. We show that an unusual response regulator, PleD, is required for polar differentiation and is sequestered to the cell pole only when it is activated by phosphorylation. Dynamic localization of PleD to the cell pole provides a mechanism to temporally and spatially control the signaling output of PleD during development. Targeting of PleD to the cell pole is coupled to the activation of a C-terminal guanylate cyclase domain, which catalyzes the synthesis of cyclic di-guanosine monophosphate. We propose that the local action of this novel-type guanylate cyclase might constitute a general regulatory principle in bacterial growth and development. During developmental transitions, localized changes of cellular morphology are mediated by adaptation in levels and arrangement of proteins. Temporal and spatial control often relies on the timed synthesis or activation of transcriptional regulators and on the establishment of gradients through the compartmentalization of signaling complexes. Although the regulatory mechanisms of gene expression are relatively well understood, it is often not clear how morphogenetic changes are controlled and coordinated locally. In prokaryotes, the major paradigm for signal transduction is the two-component regulatory system (Parkinson and Kofoid 1992). On signal input, the first component, a sensor kinase, autophosphorylates on a histidine residue. The second component, a soluble response regulator, often functions as a transcriptional regulator. Its phosphorylation by the cognate histidine kinase on a conserved aspartate residue in the N-terminal receiver domain usually results in increased DNA binding affinity (Parkinson and Kofoid 1992). Here we present evidence that a novel-type response regulator acts at a distinct subcellular site where it contributes to local changes in cell morphology through the production of a novel signaling molecule.The unicellular bacterium Caulobacter crescentus goes through an obligate developmental transition that allows it to switch between a sessile, adhesive, and a motile, planktonic cell during its cell cycle. As a consequence, cell poles are continuously remodeled during cell differentiation to facilitate assembly and removal of motility and surface adherence organelles at the right time and in the correct order. Asymmetry is established in the predivisional cell with a single flagellum, a chemotaxis machinery, and pili being assembled at one pole, whereas the opposite pole consists of a stalk and an adhesive organelle, the holdfast (Fig. 1). As a result, division generates two cell types with distinct properties: a surface-attached stalked cell and a motile swarmer cell. The swarmer progeny first differentiates into a stalked cell before it initiates DNA replication and cell division. During this transition the pili retract, flagella are released, and the adhesive organelles are synthesized at the same pole. Here we investigate the function...

Long-distance charge transport from a guanine radical cation (G(+*)) to a G-rich sequence is of biological importance. This reaction was studied by selective charge injection into a G, monitoring the charge transport to a GGG sequence by competing H(2)O-trapping. The efficiency of the charge transport diminished dramatically with increasing number of A:T base pairs between G(+*) and GGG. But in DNA strands where G's are located between the G(+*) and GGG sequence, long-distance charge transport occurred by a multistep hopping mechanism.