RELATIONSHIP BETWEEN CELL CYCLE AND LIGHT‐LIMITED GROWTH RATE IN OCEANIC PROCHLOROCOCCUS (MIT9312) AND SYNECHOCOCCUS (WH8103) (CYANOBACTERIA)1

The relationships between growth rate, cell‐cycle parameters, and cell size were examined in two unicellular cyanobacteria representative of open‐ocean environments: Prochlorococcus (strain MIT9312) and Synechococcus (strain WH8103). Chromosome replication time, C, was constrained to a fairly narrow range of values (～4–6 h) in both species and did not appear to vary with growth rate. In contrast, the pre‐ and post‐DNA replication periods, B and D, respectively, decreased with increasing growth rate from maxima of ～30 and 10–20 h to minima of ～4–6 and 2–3 h, respectively. The combined duration of the chromosome replication and postreplication periods (C+D), a quantity often used in the estimation of Prochlorococcus in situ growth rates, varied ～2.4‐fold over the range of growth rates examined. This finding suggests that assumptions of invariant C+D may adversely influence Prochlorococcus growth rate estimates. In both strains, cell mass was the greatest in slowly growing cells and decreased 2‐ to 3‐fold over the range of growth rates examined here. Estimated cell mass at the start of replication appeared to decrease with increasing growth rate, indicating that the initiation of chromosome replication in Prochlorococcus and Synechococcus is not a simple function of cell biomass, as suggested previously. Taken together, our results reflect a notable degree of similarity between oceanic Synechococcus and Prochlorococcus strains with respect to their growth‐rate‐specific cell‐cycle characteristics.     

Signs of stabilisation and stable coexistence

Many empirical studies motivated by an interest in stable coexistence have quantified negative density dependence, negative frequency dependence, or negative plant–soil feedback, but the links between these empirical results and ecological theory are not straightforward. Here, we relate these analyses to theoretical conditions for stabilisation and stable coexistence in classical competition models. By stabilisation, we mean an excess of intraspecific competition relative to interspecific competition that inherently slows or even prevents competitive exclusion. We show that most, though not all, tests demonstrating negative density dependence, negative frequency dependence, and negative plant–soil feedback constitute sufficient conditions for stabilisation of two‐species interactions if applied to data for per capita population growth rates of pairs of species, but none are necessary or sufficient conditions for stable coexistence of two species. Potential inferences are even more limited when communities involve more than two species, and when performance is measured at a single life stage or vital rate. We then discuss two approaches that enable stronger tests for stable coexistence‐invasibility experiments and model parameterisation. The model parameterisation approach can be applied to typical density‐dependence, frequency‐dependence, and plant–soil feedback data sets, and generally enables better links with mechanisms and greater insights, as demonstrated by recent studies.     

Ubiquitin‐specific protease‐like 1 (USPL1) is a SUMO isopeptidase with essential,non‐catalytic functions

Isopeptidases are essential regulators of protein ubiquitination and sumoylation. However, only two families of SUMO isopeptidases are at present known. Here, we report an activity‐based search with the suicide inhibitor haemagglutinin (HA)‐SUMO‐vinylmethylester that led to the identification of a surprising new SUMO protease, ubiquitin‐specific protease‐like 1 (USPL1). Indeed, USPL1 neither binds nor cleaves ubiquitin, but is a potent SUMO isopeptidase both in vitro and in cells. C13orf22l—an essential but distant zebrafish homologue of USPL1—also acts on SUMO, indicating functional conservation. We have identified invariant USPL1 residues required for SUMO binding and cleavage. USPL1 is a low‐abundance protein that colocalizes with coilin in Cajal bodies. Its depletion does not affect global sumoylation, but causes striking coilin mislocalization and impairs cell proliferation, functions that are not dependent on USPL1 catalytic activity. Thus, USPL1 represents a third type of SUMO protease, with essential functions in Cajal body biology.     

A sequential expression system for high-throughput functional genomic analysis

A method employing sequential rounds of cell-free protein synthesis (CFPS) was developed to identify gene products influencing the complex metabolic systems that result in protein accumulation and folding in vitro. The first round of CFPS creates an array of cell extracts individually enriched with a single gene product expressed in-parallel from linear DNA expression templates (ETs). The cell extract is engineered to enhance template stability and to provide reaction conditions conducive for general protein activation. Following first-round expression, linear templates are selectively degraded and a plasmid template for a reporter enzyme is added to initiate a subsequent round of protein expression. Reporter concentration and activity identify first-round gene products that affect amino acid and nucleic acid stability, energy supply, protein expression, stability, and activation. This sequential CFPS system provides a unique format for the functional genomic identification of broadly diverse metabolic activities.     

Structural understanding of stabilization patterns in engineered bispecific Ig‐like antibody molecules

Bispecific immunoglobulin‐like antibodies capable of engaging multiple antigens represent a promising new class of therapeutic agents. Engineering of these molecules requires optimization of the molecular properties of one of the domain components. Here, we present a detailed crystallographic and computational characterization of the stabilization patterns in the lymphotoxin‐beta receptor (LTβR) binding Fv domain of an anti‐LTβR/anti‐TNF‐related apoptosis inducing ligand receptor‐2 (TRAIL‐R2) bispecific immunoglobulin‐like antibody. We further describe a new hierarchical structure‐guided approach toward engineering of antibody‐like molecules to enhance their thermal and chemical stability. Proteins 2009. © 2009 Wiley‐Liss, Inc.     

Quantifying the evidence for ecological synergies

There is increasing concern that multiple drivers of ecological change will interact synergistically to accelerate biodiversity loss. However, the prevalence and magnitude of these interactions remain one of the largest uncertainties in projections of future ecological change. We address this uncertainty by performing a meta-analysis of 112 published factorial experiments that evaluated the impacts of multiple stressors on animal mortality in freshwater, marine and terrestrial communities. We found that, on average, mortalities from the combined action of two stressors were not synergistic and this result was consistent across studies investigating different stressors, study organisms and life-history stages. Furthermore, only one-third of relevant experiments displayed truly synergistic effects, which does not support the prevailing ecological paradigm that synergies are rampant. However, in more than three-quarters of relevant experiments, the outcome of multiple stressor interactions was non-additive (i.e. synergies or antagonisms), suggesting that ecological surprises may be more common than simple additive effects.