Molecular insights into the Darwin paradox of coral reefs from the sea anemone Aiptasia.

Symbiotic cnidarians such as corals and anemones form highly productive and biodiverse coral reef ecosystems in nutrient-poor ocean environments, a phenomenon known as Darwin's paradox. Resolving this paradox requires elucidating the molecular bases of efficient nutrient distribution and recycling in the cnidarian-dinoflagellate symbiosis. Using the sea anemone Aiptasia, we show that during symbiosis, the increased availability of glucose and the presence of the algae jointly induce the coordinated up-regulation and relocalization of glucose and ammonium transporters. These molecular responses are critical to support symbiont functioning and organism-wide nitrogen assimilation through glutamine synthetase/glutamate synthase-mediated amino acid biosynthesis. Our results reveal crucial aspects of the molecular mechanisms underlying nitrogen conservation and recycling in these organisms that allow them to thrive in the nitrogen-poor ocean environments.

Conversion of oxybenzone sunscreen to phototoxic glucoside conjugates by sea anemones and corals.

The reported toxicity of oxybenzone-based sunscreens to corals has raised concerns about the impacts of ecotourist-shed sunscreens on corals already weakened by global stressors. However, oxybenzone's toxicity mechanism(s) are not understood, hampering development of safer sunscreens. We found that oxybenzone caused high mortality of a sea anemone under simulated sunlight including ultraviolet (UV) radiation (290 to 370 nanometers). Although oxybenzone itself protected against UV-induced photo-oxidation, both the anemone and a mushroom coral formed oxybenzone-glucoside conjugates that were strong photo-oxidants. Algal symbionts sequestered these conjugates, and mortality correlated with conjugate concentrations in animal cytoplasm. Higher mortality in anemones that lacked symbionts suggests an enhanced risk from oxybenzone to corals bleached by rising temperatures. Because many commercial sunscreens contain structurally related chemicals, understanding metabolite phototoxicity should facilitate the development of coral-safe products.

Experimental investigation of tidal and freshwater influence on Symbiodiniaceae abundance in Anthopleura elegantissima.

The San Francisco Bay outflow creates a tidally influenced low-salinity plume that affects adjacent coastal sites. In the study region, Anthopleura elegantissima (Cnidaria; Anthozoa) hosts a single symbiont, the dinoflagellate Breviolum muscatinei. Salinity, temperature, and aerial stress induce a bleaching response similar to corals where symbionts are expelled, causing further energetic stress. Using field observations of environmental conditions and symbiont abundance at sites on a gradient of exposure to estuarine outflow, along with a fully crossed multifactorial lab experiment, we tested for changes in symbiont abundance in response to various combinations of three stressors. Lab experiments were designed to mimic short term outflow events with low salinity, high temperature, and aerial exposure treatments. The lab aerial exposure treatment was a statistically significant factor in suppressing symbiont repopulation (ANOVA, p = .017). In the field, symbiont density decreased with increasing tidal height at the site closest to freshwater outflow (ANOVA, p = .007), suggesting that aerial exposure may affect symbiont density more than sea surface temperature and salinity. Unanticipated documentation of survival in 9 months of sand burial and subsequent repopulation of symbionts is reported as a six-month extension to past observations, exemplifying strong tolerance to environmental insult in this Cnidarian mutualism. The study of this symbiosis is useful in examining predicted changes in ocean conditions in tidepool communities and considering relative sources of stress.

Deciphering the Roles of Multicomponent Recognition Signals by the AAA+ Unfoldase ClpX.

ATP-dependent protein remodeling and unfolding enzymes are key participants in protein metabolism in all cells. How these often-destructive enzymes specifically recognize target protein complexes is poorly understood. Here, we use the well-studied AAA+ unfoldase-substrate pair, Escherichia coli ClpX and MuA transposase, to address how these powerful enzymes recognize target protein complexes. We demonstrate that the final transposition product, which is a DNA-bound tetramer of MuA, is preferentially recognized over the monomeric apo-protein through its multivalent display of ClpX recognition tags. The important peptide tags include one at the C-terminus ("C-tag") that binds the ClpX pore and a second one (enhancement or "E-tag") that binds the ClpX N-terminal domain. We construct a chimeric protein to interrogate subunit-specific contributions of these tags. Efficient remodeling of MuA tetramers requires ClpX to contact a minimum of three tags (one C-tag and two or more E-tags), and that these tags are contributed by different subunits within the tetramer. The individual recognition peptides bind ClpX weakly (KD>70 µM) but impart a high-affinity interaction (KD~1.0 µM) when combined in the MuA tetramer. When the weak C-tag signal is replaced with a stronger recognition tag, the E-tags become unnecessary and ClpX's preference for the complex over MuA monomers is eliminated. Additionally, because the spatial orientation of the tags is predicted to change during the final step of transposition, this recognition strategy suggests how AAA+ unfoldases specifically distinguish the completed "end-stage" form of a particular complex for the ideal biological outcome.

The axial element protein HTP-3 promotes cohesin loading and meiotic axis assembly in C. elegans to implement the meiotic program of chromosome segregation.

Faithful transmission of the genome through sexual reproduction requires reduction of genome copy number during meiosis to produce haploid sperm and eggs. Meiosis entails steps absent from mitosis to achieve this goal. When meiosis begins, sisters are held together by sister chromatid cohesion (SCC), mediated by the cohesin complex. Homologs then become linked through crossover recombination. SCC subsequently holds both sisters and homologs together. Separation of homologs and then sisters requires two successive rounds of chromosome segregation and the stepwise removal of Rec8, a meiosis-specific cohesin subunit. We show that HTP-3, a known component of the C. elegans axial element (AE), molecularly links these meiotic innovations. We identified HTP-3 in a genetic screen for factors necessary to maintain SCC until meiosis II. Our data show that interdependent loading of HTP-3 and cohesin is a principal step in assembling the meiotic chromosomal axis and in establishing SCC. HTP-3 recruits all known AE components to meiotic chromosomes and promotes cohesin loading, the first known involvement of an AE protein in this process. Furthermore, REC-8 and two paralogs, called COH-3 and COH-4, together mediate meiotic SCC, but they perform specialized functions. REC-8 alone is necessary and sufficient for the persistence of SCC after meiosis I. In htp-3 and rec-8 mutants, sister chromatids segregate away from one another in meiosis I (equational division), rather than segregating randomly, as expected if SCC were completely eliminated. AE assembly fails only when REC-8, COH-3, and COH-4 are simultaneously disrupted. Premature equational sister separation in rec8 mutants of other organisms suggests the involvement of multiple REC-8 paralogs, which may have masked a conserved requirement for cohesin in AE assembly.