DNA supercoiling in Escherichia coli is under tight and subtle homeostatic control, involving gene-expression and metabolic regulation of both topoisomerase I and DNA gyrase.

DNA of prokaryotes is in a nonequilibrium structural state, characterized as 'active' DNA supercoiling. Alterations in this state affect many life processes and a homeostatic control of DNA supercoiling has been suggested [Menzel, R. & Gellert, M. (1983) Cell 34, 105-113]. We here report on a new method for quantifying homeostatic control of the high-energy state of in vivo DNA. The method involves making small perturbation in the expression of topoisomerase I, and measuring the effect on DNA supercoiling of a reporter plasmid and on the expression of DNA gyrase. In a separate set of experiments the expression of DNA gyrase was manipulated and the control on DNA supercoiling and topoisomerase I expression was measured [part of these latter experiments has been published in Jensen, P.R., van der Weijden, C.C., Jensen, L.B., Westerhoff, H.V. & Snoep, J.L. (1999) Eur. J. Biochem. 266, 865-877]. Of the two regulatory mechanisms via which homeostasis is conferred, regulation of enzyme activity or regulation of enzyme expression, we quantified the first to be responsible for 72% and the latter for 28%. The gene expression regulation could be dissected to DNA gyrase (21%) and to topoisomerase I (7%). On a scale from 0 (no homeostatic control) to 1 (full homeostatic control) we quantified the homeostatic control of DNA supercoiling at 0.87. A 10% manipulation of either topoisomerase I or DNA gyrase activity results in a 1.3% change of DNA supercoiling only. We conclude that the homeostatic regulation of the nonequilibrium DNA structure in wild-type Escherichia coli is almost complete and subtle (i.e. involving at least three regulatory mechanisms).     

Simplicity in complexity: the photosynthetic reaction center performs as a simple 0.2 V battery.

The photosynthetic reaction center is one of the most complicated molecular complexes. Transducing photon energy to a transmembrane electrochemical potential difference for protons, it is the direct or indirect energy source for virtually all life. We show here that it operates in a simple, battery-like manner, with a maximum potential of 0.20 V. Intriguingly this is only one fifth of the energy of the absorbed photon.     

The regulatory strength: How to be precise about regulation and homeostasis

The concepts of regulation and homeostasis are of frequent use but lack a single universally accepted definition. Here we propose a definition of theregulatory strength andhomeostatic strength, which allow to assess the importance of a regulatory pathway in a quantitative fashion.     

Evaluating limited specificity of drug pumps reduced relative resistance in human MDR phenotypes.

In the parallel paper, we developed a property to characterize drug efflux pumps, i.e. the reduced relative resistance (RRR). Using this RRR, we here investigate whether the observed diversity in human multidrug resistance (MDR) phenotypes might be due to variable levels of P-glycoprotein encoded by MDR1. We analyzed resistance phenotypes of various human cell lines in which either one, or both, classical human multidrug resistance genes, MDR1 and MDR3, are overexpressed. In addition, RRR values were calculated for MDR phenotypes presented in the literature. The results suggest that more than a single mechanism is required to account for the observed phenotypic diversity of classical multidrug resistance. This diversity is only partly due to differences in plasma membrane permeabilities between cell line families. It is discussed whether the alternative MDR phenotypes might be MDR1 phenotypes modified by other factors that do not themselves cause MDR. The method we here apply may also be useful for other nonspecific enzymes or pumps.     

ALGAL‐RELATED TASTES AND ODORS IN PHOENIX WATER SUPPLY: PRELIMINARY REPORT

Frequent episodes of algal‐related tastes and odors (T & O) in drinking waters in metropolitan Phoenix, Arizona prompted initiation of a three‐year project in July 1999 to investigate the occurrence of T & O metabolites and to develop a comprehensive management strategy to reduce the problems in drinking water supplies in arid environments. Two metabolites, 2‐methylisoborneol (MIB) and geosmin, have been identified as compounds responsible for the earthy‐musty tastes and odors in water supplies. Both were detected in the water supply system, including source rivers, reservoirs, canal delivery system and water treatment plants. Higher concentrations of MIB and geosmin occurred in distribution canals than in the upstream reservoirs indicating that significant production of the T & O compounds occurs within the canal system. A baseline‐monitoring program has been established for the complex water supply system, with special emphasis on the canal system. Efforts are underway to investigate possible correlations between physical/chemical parameters, algal composition and biomass, with the occurrence of MIB and geosmin. In addition, several physical and chemical treatments are planned for the canal system to reduce algal growth and related MIB and geosmin concentrations.