The photoreaction center of Rhodospirillum rubrum mutant strain F24.1

Rhodospirillum rubrum strain F24.1 is a spontaneous revertant of nonphototrophic mutant F24 derived from wild-type strain S1. Strain F24 shows no detectable photochemical activity and contains, at most, traces of the photoreaction center polypeptides. Strain F24.1 has a phototrophic growth rate close to that of the wild-type strain (Picorel, R., del Valle-Tascón, S. and Ramírez, J.M. (1977) Arch. Biophys. Biochem. 181, 665–670) but shows little photochemical activity. Light-induced absorbance changes in the near-infrared, photoinduced EPR signals and ferricyanide-elicited absorbance changes indicate that strain F24.1 has a photoreaction center content of 7–8% as compared to strain S1. Polyacrylamide gel electrophoresis of isolated F24.1 chromatophores shows the photoreaction center polypeptides to be present in amounts compatible with this value. Photoreaction center was prepared from strain F24.1 and showed no detectable difference with that of strain S1. It is concluded that strain F24.1 photosynthesis is due entirely to its residual 7–8% of typical photoreaction center.     

Role of histidine-50, glutamic acid-96, and histidine-137 in the ribonucleolytic mechanism of the ribotoxin alpha-sarcin

alpha-Sarcin is a ribotoxin secreted by the mold Aspergillus giganteus that degrades the ribosomal RNA by acting as a cyclizing ribonuclease. Three residues potentially involved in the mechanism of catalysis--histidine-50, glutamic acid-96, and histidine-137--were changed to glutamine. Three different single mutation variants (H50Q, E96Q, H137Q) as well as a double variant (H50/137Q) and a triple variant (H50/137Q/E96Q) were prepared and isolated to homogeneity. These variants were spectroscopically (circular dichroism, fluorescence emission, and proton nuclear magnetic resonance) characterized. According to these results, the three-dimensional structure of these variants of alpha-sarcin was preserved; only very minor local changes were detected. All the variants were inactive when assayed against either intact ribosomes or poly(A). The effect of pH on the ribonucleolytic activity of alpha-sarcin was evaluated against the ApA dinucleotide. This assay revealed that only the H50Q variant still retained its ability to cleave a phosphodiester bond, but it did so to a lesser extent than did wild-type alpha-sarcin. The results obtained are interpreted in terms of His137 and Glu96 as essential residues for the catalytic activity of alpha-sarcin (His137 as the general acid and Glu96 as the general base) and His50 stabilizing the transition state of the reaction catalyzed by alpha-sarcin.     

Limits to convergence of vegetation during early primary succession

Questions: Primary succession, measured by changes in species composition, is slow, usually forcing a chronose‐quence approach. A unique data set is used to explore spatial and temporal changes in vegetation structure after a 1980 volcanic eruption. On the basis of data from a transect of 20 permanent plots with an altitudinal range of 250 m sampled through 2005, two questions are asked: Do changes along the transect recapitulate succession? Do plots converge to similar composition over time? Location: A ridge between 1218 and 1468 m on Mount St. Helens, Washington, USA. Methods: Repeat sampling of plots for species cover along a 1‐km transect. Floristic changes were characterized by techniques including DCA, clustering and similarity. Results: Species richness and cover increased with time at rates that decreased with increasing elevation. The establishment of Lupinus lepidus accelerated the rate of succession and may control its trajectory. Diversity (H) at first increased with richness, then declined as dominance hierarchies developed. Primary succession was characterized by overlapping phases of species assembly (richness), vegetation maturation (diversity peaks, cover expands) and inhibition (diversity declines). Each plot passed through several community classes, but by 2005, only four classes persisted. Succession trajectories (measured by DCA) became shorter with elevation. Similarity between groups of plots defined by their classification in 2005 did not increase with time. Similarity within plot groups converged slightly at the lower elevations. Despite similarities between temporal and spatial trends in composition, trajectories of higher plots do not recapitulate those of lower plots, apparently because Lupinus was not an early colonist. Any vegetation convergence has been limited to plots that are in close proximity.     

Fitness of the pMV158 replicon in Streptococcus pneumoniae

Promiscuous, rolling-circle replication plasmid pMV158 determines tetracycline resistance to its host and can be mobilized by conjugation. Plasmid pLS1 is a deletion derivative of pMV158 that has lost its conjugative mobilization ability. Both plasmids replicate efficiently and are stably inherited in Streptococcus pneumoniae. We have analyzed the effect of pMV158 and pLS1 carriage on the bacterial growth rate. Whereas the parental plasmid does not significantly modify the cell doubling time, pLS1 slows down the growth of the bacterial host by 8-9%. The bases of the differential burden caused by pMV158 and pLS1 carriage are not yet understood. The negligible cost of the pMV158 parental natural plasmid on the host might explain the prevalence of small, multicopy, rolling-circle replication plasmids, even though they lack any selectable trait.     

Time of Progression to Osteopenia/Osteoporosis in Chronically HIV-Infected Patients: Screening DXA Scan

Background

Algorithms for bone mineral density (BMD) management in HIV-infected patients are lacking. Our objective was to assess how often a dual-energy x-ray absorptiometry (DXA) scan should be performed by assessing time of progression to osteopenia/osteoporosis.

Methods

All DXA scans performed between 2000 and 2009 from HIV-infected patients with at least two DXA were included. Time to an event (osteopenia and osteoporosis) was assessed using the Kaplan–Meier method. Strata (tertiles) were defined using baseline minimum T scores. Differences between strata in time to an event were compared with the log-rank test.

Results

Of 391 patients (1,639 DXAs), 49.6% had osteopenia and 21.7% osteoporosis at their first DXA scan. Of the 112 (28.6%) with normal BMD, 35.7% progressed to osteopenia; median progression time was 6.7 years. These patients were stratified: “low-risk" (baseline minimum T score >−0.2 SD), “middle-risk" (between −0.2 and −0.6 SD), and “high-risk" (from −0.6 to −1 SD); median progression time to osteopenia was 8.7, >7.2, and 1.7 years, respectively (p<0.0001). Of patients with osteopenia, 23.7% progressed to osteoporosis; median progression time was >8.5 years. Progression time was >8.2 years in “low-risk" tertile (T score between −1.1 and −1.6 SD), >8.5 years in “middle-risk" (between −1.6 and −2), and 3.2 years in “high-risk" (from −2 to −2.4) (p<0.0001).

Conclusions

Our results may help to define the BMD testing interval. The lowest T score tertiles would suggest recommending a subsequent DXA in 1–2 years; in the highest tertiles, ≥6 years. Early intervention in patients with bone demineralization could reduce fracture–related morbidity/mortality.     

Enhancing estimation methods for integrating probability and nonprobability survey samples with machine-learning techniques. An application to a Survey on the impact of the COVID-19 pandemic in Spain

Web surveys have replaced Face-to-Face and computer assisted telephone interviewing (CATI) as the main mode of data collection in most countries. This trend was reinforced as a consequence of COVID-19 pandemic-related restrictions. However, this mode still faces significant limitations in obtaining probability-based samples of the general population. For this reason, most web surveys rely on nonprobability survey designs. Whereas probability-based designs continue to be the gold standard in survey sampling, nonprobability web surveys may still prove useful in some situations. For instance, when small subpopulations are the group under study and probability sampling is unlikely to meet sample size requirements, complementing a small probability sample with a larger nonprobability one may improve the efficiency of the estimates. Nonprobability samples may also be designed as a mean for compensating for known biases in probability-based web survey samples by purposely targeting respondent profiles that tend to be underrepresented in these surveys. This is the case in the Survey on the impact of the COVID-19 pandemic in Spain (ESPACOV) that motivates this paper. In this paper, we propose a methodology for combining probability and nonprobability web-based survey samples with the help of machine-learning techniques. We then assess the efficiency of the resulting estimates by comparing them with other strategies that have been used before. Our simulation study and the application of the proposed estimation method to the second wave of the ESPACOV Survey allow us to conclude that this is the best option for reducing the biases observed in our data.