Isolation and characterization of a Bacillus subtilis secA mutant allele conferring resistance to sodium azide

Abstract A mutation has been isolated in the Bacillus subtilis secA gene ( secA10 ) which allows cell growth and residual protein translocation in the presence of 1.5 mM sodium azide. Besides conferring resistance to sodium azide, the corresponding SecA10 mutant protein, in which glutamic acid at position 338 has been changed to glycine, seems to possess a secretion defect even in the absence of azide. In addition, the secA10 mutant protein was found to be recessive to wild-type secA with regard to azide resistance. Our results strongly suggest that, like the situation in Escherichia coli , the B. subtilis SecA protein is a main target for the lethal action of sodium azide.     

Estimating incidence from prevalence in generalised HIV epidemics: methods and validation

Background

HIV surveillance of generalised epidemics in Africa primarily relies on prevalence at antenatal clinics, but estimates of incidence in the general population would be more useful. Repeated cross-sectional measures of HIV prevalence are now becoming available for general populations in many countries, and we aim to develop and validate methods that use these data to estimate HIV incidence.

Methods and Findings

Two methods were developed that decompose observed changes in prevalence between two serosurveys into the contributions of new infections and mortality. Method 1 uses cohort mortality rates, and method 2 uses information on survival after infection. The performance of these two methods was assessed using simulated data from a mathematical model and actual data from three community-based cohort studies in Africa. Comparison with simulated data indicated that these methods can accurately estimates incidence rates and changes in incidence in a variety of epidemic conditions. Method 1 is simple to implement but relies on locally appropriate mortality data, whilst method 2 can make use of the same survival distribution in a wide range of scenarios. The estimates from both methods are within the 95% confidence intervals of almost all actual measurements of HIV incidence in adults and young people, and the patterns of incidence over age are correctly captured.

Conclusions

It is possible to estimate incidence from cross-sectional prevalence data with sufficient accuracy to monitor the HIV epidemic. Although these methods will theoretically work in any context, we have able to test them only in southern and eastern Africa, where HIV epidemics are mature and generalised. The choice of method will depend on the local availability of HIV mortality data.     

The introduced bivalve <Emphasis Type="Italic">Limnoperna fortunei</Emphasis> boosts <Emphasis Type="Italic">Microcystis</Emphasis> growth in Salto Grande reservoir (Argentina): evidence from mesocosm experiments

In order to assess the effects of the introduced bivalve Limnoperna fortunei on water-column properties of Salto Grande reservoir, experiments were conducted using six 400 L mesocosms: 2 with 100 mussels, 2 with 300 mussels, and 2 controls (without mussels). At 0, 1, 2, 3, 7, 14, 21, 28, and 35 days we measured nutrient and chlorophyll a concentrations, counted and identified the phytoplankton, and estimated the density, size, and number of cells of the colonies of Microcystis spp. Cumulative periphyton growth and total accumulated sediments were assessed in all enclosures at the end of the experiment. Throughout the experiment, in the controls ammonia and phosphates dropped to near zero, whereas in the mesocosms with L. fortunei they increased two- to tenfold. Nitrates decreased in all mesocosms. In the presence of the mussel, chlorophyll a and algal cells dropped until day 3 increasing thereafter, whereas in the controls they increased from day 0. Periphyton growth and sediment accumulation were significantly higher in the mesocosms with mussels that in the controls. Cell density, proportion of colonial cells and colony size of Microcystis spp. increased in all enclosures, but these increases were dramatically (and very significantly) higher in enclosures with 100 and, especially, with 300 mussels, than in the controls. Our results indicate that L. fortunei modifies nutrient concentrations and proportions, and promotes aggregation of solitary Microcystis spp. cells into colonies; both these effects can favor blooms of this often noxious cyanobacteria.     

rol-Transgenic Populus tremula: root development, root-borne bud regeneration and in vitro propagation efficiency

 The potential use of the rol genes from Agrobacterium rhizogenes to improve the root system horticultural characteristics was evaluated in transgenic aspen (Populus tremula) plants, harboring the rol genes under their native promoters. Southern blot and RT-PCR analyses confirmed the presence and expression of A. rhizogenes rolC and rolB genes in four different phenotypically selected transgenic clones. Several of the observed phenotypic modifications were related to rol-gene expression and included, in particular, modified root systems. All in vitro-cultured rol-transgenic plants exhibited extensive root formation in a hormone-free medium, as well as a larger root surface area and mass, as compared to a uidA (β-glucuronidase-encoding)-transgenic aspen line and control (non-transformed) plants. Adventitious root formation in stem segments of rol-transgenic plants exhibited very rapid kinetics, resulting in a much shorter rooting time for rol-transgenic stem segments (e.g. 10 days for 80% rooting in rol-transgenic lines T-26 and T-27, as compared to more than 18 days for control non-transformed or uidA-transgenic aspen plants). rol-Transgenic plants maintained the capacity for 100% rooting throughout the year, versus 70–80% rooting in non-transformed plants during the winter. The four rol-transgenic lines exhibited differences in root development; in two of them enhanced root development was accompanied by increased shoot fresh weight. The root:shoot fresh weight ratio was always higher in rol-transgenic lines than in non-transformed plants. In the T-27 rol-transgenic line, the propagation coefficient of shoot-bud regeneration in liquid root culture was almost three times higher than in non-transformed plants. To the best of our knowledge this is the first report on quantitative phenotypic alterations in rol-transgenic woody plants. Received: 1 December 1997 / Accepted: 8 March 1998     

Phytoplankton structure and dynamics in a turbid Antarctic lake (Potter Peninsula, King George Island)

Qualitative and quantitative changes in the freshwater phytoplankton from one of the largest lakes in Potter Peninsula were studied during the summer period of 1995/1996. The main limnological features were analysed. This research constitutes the first study dealing with the phytoplankton structure and dynamics from lakes in Potter Peninsula. Due to the mixing of the water column by the constant wind action during the ice-free period, neither physical, chemical nor biological parameters showed differences between depths. Soluble reactive phosphorus remained low (26 μg l⁻¹) while total phosphorus concentrations were up to 232 μg l⁻¹. Suspended solids and total phosphorus differed significantly between the two basins in which the lake is divided. Sixty-five taxa were identified. The Bacillariophyceae was the dominant class followed by Chrysophyceae. Achnanthes lanceolata var. haynaldii and Hydrurus foetidus (cysts) were the dominant species. Tychoplanktonic species were the best represented in terms of abundance and species richness. The principal component analysis carried out with abiotic variables showed a temporal gradient characterised by the augmentation of suspended solids, total phosphorus, conductivity, pH and NH₄-N to the end of the summer period. The Principal Component Analysis performed with biotic variables also showed a temporal trend related to a decline in phytoplankton density and an enhancement of chlorophyll-a despite a slight increase of light limitation during the study period. This inverse relation may be due to the presence of picoplankton in chlorophyll-a samples. The potential effect of grazing on the phytoplankton structure is a subject to be tested in future studies. Accepted: 7 February 1999