Diversity of siderophore genes encoding biosynthesis of 2,3-dihydroxybenzoic acid in Aeromonas spp.

Most species of the genus Aeromonas produce the siderophore amonabactin, although two species produce enterobactin, the siderophore of many enteric bacteria. Both siderophores contain 2,3-dihydroxybenzoic acid (2,3-DHB). Siderophore genes (designated aebC, -E, -B and -A, for aeromonad enterobactin biosynthesis) that complemented mutations in the enterobactin genes of the Escherichia coli 2,3-DHB operon, entCEBA(P15), were cloned from an enterobactin-producing isolate of the Aeromonas spp. Mapping of the aeromonad genes suggested a gene order of aebCEBA, identical to that of the E. coli 2,3-DHB operon. Gene probes for the aeromonad aebCE genes and for amoA (the entC-equivalent gene previously cloned from an amonabactin-producing Aeromonas spp.) did not cross-hybridize. Gene probes for the E. coli 2,3-DHB genes entCEBA did not hybridize with Aeromonas spp. DNA. Therefore, in the genus Aeromonas, 2,3-DHB synthesis is encoded by two distinct gene groups; one (amo) is present in the amonabactin-producers, while the other (aeb) occurs in the enterobactin-producers. Each of these systems differs from (but is functionally related to) the E. coli 2,3-DHB operon. These genes may have diverged from an ancestral group of 2,3-DHB genes.     

Free and polymerized tubulin in cultured bone cells and Chinese hamster ovary cells: the influence of cold and hormones

A low pH method of liposome-membrane fusion (Schneider et al., 1980, Proc. Natl. Acad. Sci. U. S. A. 77:442) was used to enrich the mitochondrial inner membrane lipid bilayer 30-700% with exogenous phospholipid and cholesterol. By varying the phospholipid-to- cholesterol ratio of the liposomes it was possible to incorporate specific amounts of cholesterol (up to 44 mol %) into the inner membrane bilayer in a controlled fashion. The membrane surface area increased proportionally to the increase in total membrane bilayer lipid. Inner membrane enriched with phospholipid only, or with phospholipid plus cholesterol up to 20 mol %, showed randomly distributed intramembrane particles (integral proteins) in the membrane plane, and the average distance between intramembrane particles increased proportionally to the amount of newly incorporated lipid. Membranes containing between 20 and 27 mol % cholesterol exhibited small clusters of intramembrane particles while cholesterol contents above 27 mol % resulted in larger aggregations of intramembrane particles. In phospholipid-enriched membranes with randomly dispersed intramembrane particles, electron transfer activities from NADH- and succinate-dehydrogenase to cytochrome c decreased proportionally to the increase in distance between the particles. In contrast, these electron- transfer activities increased with decreasing distances between intramembrane particles brought about by cholesterol incorporation. These results indicate that (a) catalytically interacting redox components in the mitochondrial inner membrane such as the dehydrogenase complexes, ubiquinone, and heme proteins are independent, laterally diffusible components; (b) the average distance between these redox components is effected by the available surface area of the membrane lipid bilayer; and (c) the distance over which redox components diffuse before collision and electron transfer mediates the rate of such transfer.     

Do artificial substrates favor nonindigenous fouling species over native species?

Exotic species are prominent constituents of fouling communities. If exotic fouling organisms colonize or compete better on a wider range of substrate types than native species, this may partially account for their high abundance in estuaries and bays. We used four artificial and four naturally occurring substrate types to compare initial settlement and percent cover of native and exotic fouling species through six months of community development. Both the identity of common taxa and the total number of species colonizing artificial versus natural substrate types were similar. Despite the similarities in species richness, relative abundance patterns between natural and artificial substrate types varied, particularly as the communities developed. Native species were initially in equal abundance on natural and artificial substrate types. Initially, the two most common exotic species, the colonial tunicates, Botrylloides violaceus Ritter and Forsyth and Botryllus schlosseri (Pallas), were also in similar, but low, abundance on artificial and natural substrates. As the communities developed, there was little change in abundance of exotic or native species on natural substrates. However, on artificial substrates the exotic tunicates increased dramatically and native species declined in abundance. Artificial surfaces may provide a novel context for competitive interactions giving exotic species a more “level playing field” in an environment for which they otherwise might not be as well adapted compared to long-resident native species. Additions of artificial substrates to nearshore environments may disproportionately favor exotic species by increasing local sources of exotic propagules to colonize all types of substrates.     

Community impacts of two invasive crabs: the interactive roles of density, prey recruitment, and indirect effects

Assessing the implications of species invasion for native communities requires determining whether effects of invaders are novel, or are redundant with effects of species that are already present. Using a pair of field experiments conducted over two successive years, we examined factors that influence community impacts of a recent predatory crab invader (Hemigrapsus sanguineus) and a previously established invasive crab (Carcinus maenas) on New England coasts. We demonstrate that effects of these species differ temporally with changes in the ambient prey community, and are influenced by density differences between the two species and by different strengths and types of indirect effects that each elicits. Our study highlights the importance of including bottom-up processes (i.e., prey recruitment) when examining the redundancy of consumers.     

Insertional Mutations in the Yeast Hop1 Gene: Evidence for Multimeric Assembly in Meiosis

The HOP1 gene of Saccharomyces cerevisiae has been shown to play an important role in meiotic synapsis. In this study we analyzed the mechanism of this function by phenotypic characterization of novel in-frame linker-insertion mutations located at various sites throughout the HOP1 coding sequence. Among 12 mutations found to cause defects in meiotic recombination and spore viability, three were temperature-sensitive for the spore viability defect. Although substantial meiotic recombination was found for these conditional alleles at the restrictive temperature, the level of exchange measured in spo13 meiosis was reduced in some of the monitored intervals, indicating that nondisjunction resulting from a deficit in crossing over could account for SPO13 spore inviability. Intragenic complementation between linker-insertion alleles was assessed by testing the viability of spores generated from heteroallelic diploids after SPO13 meiosis. Complex patterns of complementation and enhancement of the spore-inviability phenotype indicate that HOP1 functions in a multimeric complex. In addition, the ability of alleles which map near the carboxyl terminus to complement several other alleles provides evidence for a functional domain in this region of the protein. Two previously identified multicopy suppressors of the conditional hop1-628(ts) allele were tested for their effects in cells bearing the linker-insertion hop1 alleles. Overexpression of REC104 from a 2μ plasmid was shown to enhance the spore viability of every allele tested, including a hop1 disruption allele. On the other hand, suppression by overexpression of RED1 from a 2μ plasmid was found only for allele hop1-628(ts). Surprisingly, similar overexpression of RED1 in strains bearing several other conditional hop1 linker-insertion alleles caused enhanced spore lethality. This finding, in conjunction with the evidence for a carboxy-terminal domain, provides new insight into the nature of interactions between the HOP1 and RED1 products in meiosis.