Acridine Orange, a precipitant for sulfated glycosaminoglycans, causes mucopolysaccharidosis in cultured fibroblasts

Modulation of Lrp action in Escherichia coli by leucine: effects on non-specific binding of Lrp to DNA

Lrp is a global regulator of metabolism in Escherichia coli that helps cells respond to changes in environmental conditions. The action of Lrp as a transcriptional activator or repressor is sometimes affected by whether the medium contains exogenous leucine. The abundance of Lrp in cells is relatively high (about 15 microM in monomer), and given the relatively high Lrp binding affinity in vitro for specific binding sites (nanomolar apparent dissociation constants), the expectation is that all binding sites will be saturated with Lrp in vivo. Here we consider the fraction of the total Lrp in cells that is free and the fraction that is bound to DNA. Using minicell-producing strains, we measured the distribution of Lrp between cytoplasm and nucleoid in cells grown under different nutritional conditions and in cells in different phases of growth. In E. coli cells grown in minimal medium to mid-log phase, the ratio of free to DNA-bound Lrp was about 0.67. This ratio decreased about threefold when the cells were grown in minimal medium supplemented with leucine. Our results also confirmed the previous finding that growth rate regulates lrp expression by as much as three to fourfold. Growth rate-regulated lrp expression, along with changes in the extent of non-specific binding, influences the level of free Lrp in vivo over a 16-fold range. We propose that the net effect of these processes is to regulate the relative concentrations of free Lrp hexadecamer and leucine-bound octamer, leading to promoter selection in response to environmental conditions.

Co-localization of hydrolytic enzymes with widely disparate pH optima: implications for the regulation of lysosomal pH

Lysosomes are traditionally defined by their acidic interior, their content of degradative ‘acid hydrolases’, and the presence of distinctive membrane proteins. Terminal degradation of the N-linked oligosaccharides of glycoproteins takes place in lysosomes, and involves several hydrolases, many of which are known to have acidic pH optima. However, a sialic acid-specific 9-O-acetyl-esterase and a glycosyl-N-asparaginase, which degrade the outer- and inner-most linkages of N-linked oligosaccharides, respectively, both have pH optima in the neutral to alkaline range. By immunoelectron microscopy, these enzymes co-localize in lysosomes with several conventional acid hydrolases and with lysosomal membrane glycoproteins. Factors modifying the pH/activity profiles of these enzymes could not be found in lysosomal extracts. Thus, the function of the enzymes with neutral pH optima must depend either upon their minimal residual activity at acidic pH, or upon the possibility that lysosomes are not always strongly acidic. Indeed, when lysosomes are marked in living cells by uptake of fluorescently labeled mannose 6-phosphorylated proteins, the labeled organelles do not all rapidly accumulate Acridine Orange, a vital stain that is specific for acidic compartments. One plausible explanation is that lysosomal pH fluctuates, allowing hydrolytic enzymes with a wide range of pH optima to efficiently degrade macromolecules.