Epistatic effects of promoter and repressor functions of the Tn10 tetracycline-resistance operon on the fitness of Escherichia coli

We have been studying the effects of expression of plasmid-borne, Tn10-encoded, tetracycline resistance on the fitness of Escherichia coli K12. We previously demonstrated large reductions in fitness resulting from induced or constitutive expression of the resistance protein; however, any residual expression by the repressed operon was so slight that possession of an inducible resistance function imposed essentially no burden in the absence of antibiotic. Here, we demonstrate two distinct disadvantages for inducible genotypes relative to isogenic constitutive constructs. During the transition from antibiotic-free to antibiotic-containing media, the inducible genotype experiences a longer lag phase prior to growth. In the sustained presence of antibiotic, full induction of the resistance function in the inducible genotype is prevented by the continued action of its repressor. However, these disadvantages may be reduced by increasing the strength of the promoter for the resistance gene in the inducible genotype. Simultaneous consideration of the mode of gene regulation (i.e. constitutive or inducible) and the strength of the resistance-gene promoter (i.e. maximum level of expression) indicates an adaptive landscape with very strong epistasis and, perhaps, multiple fitness peaks.     

Influence of Pneumocystis carinii on Nitrite Production by Rat Alveolar Macrophages1

ABSTRACT Nitrite production by rat alveolar macrophages was studied to determine the role of L-arginine oxidation in the interaction between these cells and Pneumocystis carinii. Alveolar macrophages from rats obtained from two different breeders were used: rats from Janvier breeder had latent P. carinii infection, while those from Charles River breeder were bred in a germ-free environment. Pneumocystis carinii increased in vitro nitrite generation by unstimulated alveolar macrophages from Janvier rats only, and this was blocked by N^G-monomethyl-L-arginine. Incubation of cells from Janvier and Charles River rats with lipopolysaccharide and/or interferon-gamma increased nitrite production to a similar extent. Pneumocystis carinii partially decreased nitrite release by activated alveolar macrophages, and this was still inhibited by N^G-monomethyl-L-arginine. In the presence of P. carinii, superoxide dismutase used as a superoxide anion scavenger had no effect on nitrite production by activated cells. These results show that prior exposure to P. carinii leads to nitric oxide production by rat alveolar macrophages. Although the magnitude of this production seems to be moderate, it is of biological significance since cells of P. curinii-naive rats do not generate nitrite whereas those of latently infected rats do.     

Morph-specific patterns of variation in stigma height in natural populations of distylous Jasminum fruticans

The purpose of this study was to describe morph-specific patterns of stigma–anther separation and style curling in the distylous Jasminum fruticans . We also examined whether variation in floral traits is correlated with variation in seed production. Stigma–anther separation is more variable in short-styled plants than in long-styled plants. In all populations studied, some plants bear flowers that lack any stigma–anther separation. These plants have the pollen characteristics (size and number) and compatibility relations of short-styled plants. Comparison with other distylous species illustrates that the variability of stigma–anther separation in short-styled plants is a novel finding for a distylous species. Long-styled plants have greater stigma–anther separation than short-styled plants, styles are often curled and protrude from the corolla, and anthers are placed well within the corolla tube. The frequency of long-styled plants with curled styles and the mean degree of style curling were significantly correlated with style length. Short-styled plants have larger corollas than long-styled plants and never have curled styles. Morph ratios are always 50:50 in natural populations. Mean values of each floral trait in the two morphs were significantly correlated among populations. There were no consistent differences in fecundity of the two morphs nor any correlation between floral traits and seed set for each morph in natural populations. We discuss the potential causes and significance of the two morph-specific patterns we describe; reduced stigma–anther separation in short-styled plants and the presence of curled styles in long-styled plants.