The actin cytoskeleton and cytoplasmic intermediate filaments contribute to cell migration and morphogenesis, but the interplay between these two central cytoskeletal elements has remained elusive. Here, we find that specific actin stress fiber structures, transverse arcs, interact with vimentin intermediate filaments and promote their retrograde flow. Consequently, myosin-II-containing arcs are important for perinuclear localization of the vimentin network in cells. The vimentin network reciprocally restricts retrograde movement of arcs and hence controls the width of flat lamellum at the leading edge of the cell. Depletion of plectin recapitulates the vimentin organization phenotype of arc-deficient cells without affecting the integrity of vimentin filaments or stress fibers, demonstrating that this cytoskeletal cross-linker is required for productive interactions between vimentin and arcs. Collectively, our results reveal that plectin-mediated interplay between contractile actomyosin arcs and vimentin intermediate filaments controls the localization and dynamics of these two cytoskeletal systems and is consequently important for cell morphogenesis.

In late summer, nitrogen-fixing cyanobacteria Nodularia spumigena and Aphanizomenon flos-aquae form blooms in the open Baltic Sea. N. spumigena has caused several animal poisonings, but Baltic A. flos-aquae is not known to be toxic. In this laboratory study, performed with batch cultures, the influences of environmental conditions on the biomass and nitrogen fixation rate of N. spumigena and A. flos-aquae were compared and the toxin (nodularin) concentration produced by N. spumigena was measured. Several differences in the biomasses and nitrogen fixation rates of N. spumigena and A. flos-aquae were observed. A. flos-aquae preferred lower irradiances, salinities, and temperatures than N. spumigena. The biomass of both species increased with high phosphate concentrations and with accompanying bacteria and decreased with unnaturally high inorganic nitrogen concentrations. Nodularin concentrations in cells and growth media, as well as nitrogen fixation rates, were generally highest under the conditions that promoted growth. Intracellular nodularin concentrations increased with high temperature, high irradiance, and high phosphate concentration and decreased with low and high salinities and high inorganic nitrogen concentrations. Nodularin concentrations in growth media increased with incubation time, indicating that intracellular nodularin was released when cells lysed. The different responses of A. flos-aquae and N. spumigena to changes in salinity, irradiance, and temperature may explain the different spatial and temporal distribution of these species in the Baltic Sea. According to the results, toxic N. spumigena blooms may be expected in late summer in areas of the Baltic Sea with high phosphorus concentrations and moderate salinity.

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An investigation was undertaken of the genetic diversity of Nodularia strains from the Baltic Sea and from Australian waters, together with the proposed type strain of Nodularia spumigena. The Nodularia strains were characterized by using a polyphasic approach, including RFLP of PCR-amplified 16S rRNA genes, 16S rRNA gene sequencing, Southern blotting of total DNA, repetitive extragenic palindromic-and enterobacterial repetitive intergenic consensus-PCR, ribotyping and phenotypic tests. With genotypic methods, the Nodularia strains were grouped into two clusters. The genetic groupings were supported by one phenotypic property : the ability to produce nodularin. In contrast, the cell sizes of the strains were not different in the two genetic clusters. 16S rRNA gene sequences indicated that all the Nodularia strains were closely related, despite their different origins. According to this study, two genotypes of Nodularia exist in the Baltic Sea. On the basis of the taxonomic definitions of Koma! rek et al. (Algol Stud 68, 1-25, 1993), the non-toxic type without gas vesicles fits the description of Nodularia sphaerocarpa, whereas the toxic type with gas vesicles resembles the species N. spumigena and Nodularia baltica.Keywords : Nodularia cyanobacterium, RFLP, 16S rDNA sequencing, REP-and ERIC-PCR, ribotyping INTRODUCTIONThe cyanobacterial genus Nodularia commonly form blooms in brackish waters such as the Baltic Sea in Europe (Sivonen et al., 1989a ;Kononen et al., 1993) and in coastal lagoons and brackish water lakes in Australia (Baker & Humpage, 1994 ;Jones et al., 1994). These blooms have caused numerous cases of animal poisoning (Francis, 1878 ;Nehring, 1993) due to a hepatotoxin called nodularin (Rinehart et al., 1988). It is a small, cyclic pentapeptide and has an LD &! of 50-70 µg kg −" when administered intra- peritoneally in mouse (Rinehart et al., 1988 ; Sivonen et al., 1989a). The toxicity of nodularin results from its ability to inhibit the serine\threonine-specific protein phosphatases PP1 and PP2A (Ohta et al., 1994). Therefore, nodularin is a potent tumour promoter and a possible carcinogen (Ohta et al., 1994). Nodularin is produced continuously by some Nodularia strains (Lehtima$ ki et al., 1994(Lehtima$ ki et al., , 1997.Cyanobacteria have traditionally been classified on the basis of morphology. The analysis of 16S rRNA genes has revealed that morphological characters do not necessarily result in a phylogenetically reliable taxonomy (Giovannoni et al., 1988 ;Wilmotte, 1994). On the basis of morphological criteria, Walsby et al. (1995), Albertano et al. (1996), Hayes & Barker (1997) and Barker et al. (1999) found four different morphological types of Nodularia together in the Baltic Sea. One of them was not identified to the species level, whereas three were distinguished according to the criteria of Koma! rek et al. (1993) Koma! rek et al. (1993). The species were differentiated by ecology and morphology such as the presence of gas vesicles, the dimensions and shapes of vegetative cells, hetero...