The phytochrome family of plant photoreceptors has a central role in the adaptation of plant development to changes in ambient light conditions. The individual phytochrome species regulate different or partly overlapping physiological responses. We generated transgenic Arabidopsis plants expressing phytochrome A to E:green fluorescent protein (GFP) fusion proteins to assess the biological role of intracellular compartmentation of these photoreceptors in lightregulated signaling. We show that all phytochrome:GFP fusion proteins were imported into the nuclei. Translocation of these photoreceptors into the nuclei was regulated differentially by light. Light-induced accumulation of phytochrome species in the nuclei resulted in the formation of speckles. The appearance of these nuclear structures exhibited distinctly different kinetics, wavelengths, and fluence dependence and was regulated by a diurnal rhythm. Furthermore, we demonstrate that the import of mutant phytochrome B:GFP and phytochrome A:GFP fusion proteins, shown to be defective in signaling in vivo, is regulated by light but is not accompanied by the formation of speckles. These results suggest that (1) the differential regulation of the translocation of phytochrome A to E into nuclei plays a role in the specification of functions, and (2) the appearance of speckles is a functional feature of phytochrome-regulated signaling.
Little is known about the division of eukaryotic cell organelles and up to now neither in animals nor in plants has a gene product been shown to mediate this process. A cDNA encoding a homolog of the bacterial cell division protein FtsZ, an ancestral tubulin, was isolated from the eukaryote Physcomitrella patens and used to disrupt efficiently the genomic locus in this terrestrial seedless plant. Seven out of 51 transgenics obtained were knockout plants generated by homologous recombination; they were specifically impeded in plastid division with no detectable effect on mitochondrial division or plant morphology. Implications on the theory of endosymbiosis and on the use of reverse genetics in plants are discussed.
No abstract
Pathogenicity of the human pathogen Helicobacter pylori relies upon its capacity to adapt to a hostile environment and to escape from the host response. Therefore, cell shape, motility, and pH homeostasis of these bacteria are specifically adapted to the gastric mucus. We have found that the helical shape of H. pylori depends on coiled coil rich proteins (Ccrp), which form extended filamentous structures in vitro and in vivo, and are differentially required for the maintenance of cell morphology. We have developed an in vivo localization system for this pathogen. Consistent with a cytoskeleton-like structure, Ccrp proteins localized in a regular punctuate and static pattern within H. pylori cells. Ccrp genes show a high degree of sequence variation, which could be the reason for the morphological diversity between H. pylori strains. In contrast to other bacteria, the actin-like MreB protein is dispensable for viability in H. pylori, and does not affect cell shape, but cell length and chromosome segregation. In addition, mreB mutant cells displayed significantly reduced urease activity, and thus compromise a major pathogenicity factor of H. pylori. Our findings reveal that Ccrp proteins, but not MreB, affect cell morphology, while both cytoskeletal components affect the development of pathogenicity factors and/or cell cycle progression.
Mycoplasma pneumoniae cells adhering to glass or Parlodion-coated grids were extracted with Triton X-100. The extracted cells showed a cytoskeleton consisting of a rodlike tip structure and a filamentous network in the cytoplasm. The tip structure was up to 300 nm long and -40 nm wide ending at the distal end in a bleb-like structure, and seemed to consist of filaments arranged in parallel, 4.8 ± 0.5 nm wide . In the cytoplasm the filaments formed an irregular lattice. Similar filaments were found in platinum replicated critical-point dried extracted cells. An actinlike nature of the filaments is suggested by some of their properties, but the degree of homology with respect to eucaryotic actin is still unknown . The filaments were sensitive to protease treatment but stable in high molar KCI solutions . They were apparently destroyed by incubation in high molar KI solution, leaving only some parts of the tip structure. Formaldehyde-fixed M. pneumoniae cells treated with Triton X-100 bound rhodamine-labeled phalloidin specifically . Furthermore, they could be stained with antiactin antibodies . Binding of myosin subfragment 1 to the filaments was not observed .In animal and plant cells, microfilaments are involved in motile processes such as cell contraction, cytoplasmic streaming, and pseudopod or filopod formation during amoeboid movement, Information about the arrangement of microfilaments within the cell, their biochemical characteristics, and their role in maintenance of cell shape and cell motility is accumulating (1,25,26) .In contrast to eucaryotic cells, the molecular basis of motility in procaryotic cells is only poorly understood . In the mycoplasmas, the smallest procaryotes known to grow in a cell-free medium, several observations suggest the presence of contractile elements : (a) specialized tip structures and gliding movement in the species M. gallisepticum, M. pneumoniae, and M. pulmonis (7, 9, 27), (b) contractile processes in M . hominis and M. orale, as revealed in microcinematographic studies (7, 8), (c) the possible presence of actin-like proteins in several species including Spiroplasma citri (6,21,24,32).In the present study we use M. pneumoniae to demonstrate the presence and the in situ arrangement of actin-like microfilaments. MATERIALS AND METHODS MycoplasmasU . GOBEL, V. SPETH, and W. BREDT Institute for General Hygiene and Bacteriology, Center for Hygiene, University of Freiburg, and MaxPlanck-Institute for Immunobiology, Freiburg, Federal Republic of Germany M. pneumoniae strain FH was grown in a modified Hayflick medium containing 20% inactivated horse serum, 0.1% glucose, 0.05% thallium acetate and 1000 U/ml penicillin (27) .
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