Holger Hartmann

Streptomyces coelicolor A3(2) synthesizes ectoine and 5-hydroxyectoine upon the imposition of either salt (0.5 M NaCl) or heat stress (39 degrees C). The cells produced the highest cellular levels of these compatible solutes when both stress conditions were simultaneously imposed. Protection against either severe salt (1.2 M NaCl) or heat stress (39 degrees C) or a combination of both environmental cues could be accomplished by adding low concentrations (1 mM) of either ectoine or 5-hydroxyectoine to S. coelicolor A3(2) cultures. The best salt and heat stress protection was observed when a mixture of ectoine and 5-hydroxyectoine (0.5 mM each) was provided to the growth medium. Transport assays with radiolabeled ectoine demonstrated that uptake was triggered by either salt or heat stress. The most effective transport and accumulation of [(14)C]ectoine by S. coelicolor A3(2) were achieved when both environmental cues were simultaneously applied. Our results demonstrate that the accumulation of the compatible solutes ectoine and 5-hydroxyectoine allows S. coelicolor A3(2) to fend off the detrimental effects of both high salinity and high temperature on cell physiology. We also characterized the enzyme (EctD) required for the synthesis of 5-hydroxyectoine from ectoine, a hydroxylase of the superfamily of the non-heme-containing iron(II)- and 2-oxoglutarate-dependent dioxygenases (EC 1.14.11). The gene cluster (ectABCD) encoding the enzymes for ectoine and 5-hydroxyectoine biosynthesis can be found in the genome of S. coelicolor A3(2), Streptomyces avermitilis, Streptomyces griseus, Streptomyces scabiei, and Streptomyces chrysomallus, suggesting that these compatible solutes play an important role as stress protectants in the genus Streptomyces.

Environmental signals induce and coordinate discrete morphological transitions during sexual development of Ustilago maydis. In this fungus, mating of two compatible haploid sporidia is a prerequisite for plant infection. Cell fusion is governed by the action of pheromones and receptors, whereas the subsequent pathogenicity program is controlled by the combinatorial interaction of homeodomain proteins. The U. maydis pheromone response factor (Prf1) is a central regulator of both processes. We have analyzed the regulation of the prf1 gene and demonstrate that pheromone and cAMP signaling regulate prf1 post-transcriptionally. Transcriptional activation of prf1 was observed in the presence of carbon sources, such as glucose and fructose, allowing us to define the cis-acting element in the prf1 promoter that mediates these effects. The same element provides for negative control of prf1 gene transcription at high cAMP levels. A protein that specifically binds to this element was purified and analyzed for its role in prf1 gene regulation. On the basis of these results, we present a model in which prf1 integrates different environmental signals to control development in U. maydis.