SummaryWe have constructed a matched set of binary vectors designated pGD, pGDG and pGDR for the expression and co-localization of native proteins and GFP or DsRed fusions in large numbers of plant cells. The utility of these vectors following agroin®ltration into leaves has been demonstrated with four genes from Sonchus yellow net virus, a plant nucleorhabdovirus, and with a nucleolar marker protein.Of the three SYNV proteins tested, sc4 gave identical localization patterns at the cell wall and nucleus when fused to GFP or DsRed. However, some differences in expression patterns were observed depending on whether DsRed or GFP was the fusion partner. In this regard, the DsRed:P fusion showed a similar pattern of localization to GFP:P, but localized foci appeared in the nucleus and near the periphery of the nucleus. Nevertheless, the viral nucleocapsid protein, expressed as a GFP:N fusion, colocalized with DsRed:P in a subnuclear locale in agreement with our previous observations (Goodin et al., 2001). This locale appears to be distinct from the nucleolus as indicated by co-expression of the N protein, DsRed:P and a nucleolar marker AtFib1 fused to GFP. The SYNV M protein, which is believed to be particularly prone to oligomerization, was detectable only as a GFP fusion. Our results indicate that agroin®ltration with bacteria containing the pGD vectors is extremely useful for transient expression of several proteins in a high proportion of the cells of Nicotiana benthamiana leaves. The GFP and DsRed elements incorporated into the pGD system should greatly increase the ease of visualizing colocalization and interactions of proteins in a variety of experimental dicotyledonous hosts.

Despite their importance in iron redox cycles and bioenergy production, the underlying physiological, genetic, and biochemical mechanisms of extracellular electron transfer by Gram-positive bacteria remain insufficiently understood. In this work, we investigated respiration by Thermincola potens strain JR, a Grampositive isolate obtained from the anode surface of a microbial fuel cell, using insoluble electron acceptors. We found no evidence that soluble redox-active components were secreted into the surrounding medium on the basis of physiological experiments and cyclic voltammetry measurements. Confocal microscopy revealed highly stratified biofilms in which cells contacting the electrode surface were disproportionately viable relative to the rest of the biofilm. Furthermore, there was no correlation between biofilm thickness and power production, suggesting that cells in contact with the electrode were primarily responsible for current generation. These data, along with cryo-electron microscopy experiments, support contact-dependent electron transfer by T. potens strain JR from the cell membrane across the 37-nm cell envelope to the cell surface. Furthermore, we present physiological and genomic evidence that c-type cytochromes play a role in charge transfer across the Gram-positive bacterial cell envelope during metal reduction.

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We have now used Agrobacterium tumefaciens-mediated protein expression in Nicotiana benthamiana leaf cells and site-specific mutagenesis to determine how TGB protein interactions influence their subcellular localization and virus spread. Confocal microscopy revealed that the TGB3 protein localizes at the cell wall (CW) in close association with plasmodesmata and that the deletion or mutagenesis of a single amino acid at the immediate C terminus can affect CW targeting. TGB3 also directed the localization of TGB2 from the endoplasmic reticulum to the CW, and this targeting was shown to be dependent on interactions between the TGB2 and TGB3 proteins. The optimal localization of the TGB1 protein at the CW also required TGB2 and TGB3 interactions, but in this context, site-specific TGB1 helicase motif mutants varied in their localization patterns. The results suggest that the ability of TGB1 to engage in homologous binding interactions is not essential for targeting to the CW. However, the relative expression levels of TGB2 and TGB3 influenced the cytosolic and CW distributions of TGB1 and TGB2. Moreover, in both cases, localization at the CW was optimal at the 10:1 TGB2-to-TGB3 ratios occurring in virus infections, and mutations reducing CW localization had corresponding effects on BSMV movement phenotypes. These data support a model whereby TGB protein interactions function in the subcellular targeting of movement protein complexes and the ability of BSMV to move from cell to cell.

Plant rhabdoviruses are classified into the genera Cytorhabdovirus and Nucleorhabdovirus on the basis of their sites of replication, morphogenesis, and maturation (for a review, see reference 17). Sonchus yellow net nucleorhabdovirus (SYNV) replicates in the nucleus and is the most extensively characterized among the plant rhabdoviruses. SYNV encodes six genes in a negative-sense orientation; these six genes encode a nucleocapsid protein (N), a phosphoprotein (P), a putative movement protein (sc4), a matrix protein (M), a glycoprotein (G), and a large polymerase protein (L). The N, P, and L proteins are components of an infectious nucleocapsid core (15) with RNA-dependent RNA polymerase activity that can be purified from the nuclei of virus-infected cells (34,35). These core components form viroplasm-like structures within the nucleus that are thought to be the sites of viral replication (22). The N protein contains a carboxy (C)-terminal bipartite nuclear localization signal (NLS) that is required for nuclear import (7). The P protein when expressed alone localizes both inside and outside of the nucleus, but coexpression of the N and P proteins in plant and yeast (Saccharomyces cerevisiae) cells results in formation of compact subnuclear foci that are reminiscent of viroplasms (7). Sedimentation and immunological analyses have shown that in vivo associations of the N, P, and L proteins are required for RNA-dependent RNA polymerase activity (35). Yeast two-hybrid analyses and affinity chromatography experiments have also verified homologous (N-N) and heterologous (N-P) binding that is mediated by a region near the amino (N) terminus of the N protein (7). These results indicate that complex interactions of the N, P, and L proteins are required for subnuclear viroplasm formation and that the nucleocapsid cores within the viroplasms function in replication of genomic and antigenomic RNAs and in mRNA transcription (17).In the current study, we have conducted experiments to define the contributions of the N protein to the formation of viroplasms. These experiments include refined mapping to identify amino acids in an N-terminal helix-loop-helix motif of the N protein that result in subnuclear localization and N-N and N-P protein binding. We have also shown that mutations within the helix-loop-helix motif that disrupt N-N and N-P interactions interfere with the formation of subnuclear foci. The C-terminal NLS that is required for nuclear import of the N protein (7) mediates binding to importin ␣ homologues from yeast (ySrp1) and plant Arabidopsis thaliana (AtSrp1), but the P protein fails to bind to the importin ␣ homologues. These