Effect of dibutyryl cyclic adenosine monophosphate on active amino acid transport in Streptomyces hydrogenans

Primary metabolism and its control in streptomycetes: a most unusual group of bacteria

Streptomycetes are Gram-positive bacteria with a unique capacity for the production of a multitude of varied and complex secondary metabolites. They also have a complex life cycle including differentiation into at least three distinct cell types. Whilst much attention has been paid to the pathways and regulation of secondary metabolism, less has been paid to the pathways and the regulation of primary metabolism, which supplies the precursors. With the imminent completion of the total genome sequence of Streptomyces coelicolor A3(2), we need to understand the pathways of primary metabolism if we are to understand the role of newly discovered genes. This review is written as a contribution to supplying these wants. Streptomycetes inhabit soil, which, because of the high numbers of microbial competitors, is an oligotrophic environment. Soil nutrient levels reflect the fact that plant-derived material is the main nutrient input; i.e. it is carbon-rich and nitrogen- and phosphate-poor. Control of streptomycete primary metabolism reflects the nutrient availability. The variety and multiplicity of carbohydrate catabolic pathways reflects the variety and multiplicity of carbohydrates in the soil. This multiplicity of pathways has led to investment by streptomycetes in pathway-specific and global regulatory networks such as glucose repression. The mechanism of glucose repression is clearly different from that in other bacteria. Streptomycetes feed by secreting complexes of extracellular enzymes that break down plant cell walls to release nutrients. The induction of these enzyme complexes is often coordinated by inducers that bear no structural relation to the substrate or product of any particular enzyme in the complex; e.g. a product of xylan breakdown may induce cellulase production. Control of amino acid catabolism reflects the relative absence of nitrogen catabolites in soil. The cognate amino acid induces about half of the catabolic pathways and half are constitutive. There are reduced instances of global carbon and nitrogen catabolite control of amino acid catabolism, which again presumably reflects the relative rarity of the catabolites. There are few examples of feedback repression of amino acid biosynthesis. Again this is taken as a reflection of the oligotrophic nature of the streptomycete ecological niche. As amino acids are not present in the environment, streptomycetes have rarely invested in feedback repression. Exceptions to this generalization are the arginine and branched-chain amino acid pathways and some parts of the aromatic amino acid pathways which have regulatory systems similar to Escherichia coli and Bacillus subtilis and other copiotrophic bacteria.

Control of actinomycin D biosynthesis in Streptomyces parvullus: regulation of tryptophan oxygenase activity

Tryptophan oxygenase (tryptophan 2,3-dioxygenase) activity increases immediately before the initiation of actinomycin D production by Streptomyces parvullus. We have attempted to discern whether this increase is due to a release from catabolite repression or to the synthesis of an inducer substance. The standard culture medium (glutamic acid-histidine-fructose medium) used in antibiotic production studies with S. parvullus contains l-glutamate as a major constituent. l-Glutamate is almost totally consumed before the onset of actinomycin D synthesis. The addition of 10 mM l-glutamate at this stage completely abolished actinomycin D production as well as tryptophan oxygenase synthesis. Fourteen amino acids were tested for a similar effect. Of these, l-glutamate and l-aspartate had the most dramatic effect on tryptophan oxygenase and beta-galactosidase (beta-d-galactosidase), another inducible enzyme. Standard glutamic acid-histidine-fructose medium, preincubated for 23 h to remove l-glutamate, allowed the synthesis of actinomycin D and tryptophan oxygenase by cells at a stage of growth normally considered too early for antibiotic production. A chemically defined medium lacking l-glutamate and adjusted to pH 8.0 was designed to simulate the preincubation medium. The transfer of cells to this artificial preincubation medium resulted in the appearance of tryptophan oxygenase as early as 19 h before normal synthesis occurred, eliminating the possibility that an inducer molecule is synthesized and excreted during the preincubation period. The results of these studies suggest that the increase in tryptophan oxygenase activity before the onset of actinomycin D synthesis, as well as the synthesis of actinomycin D itself, is due to a release from l-glutamate catabolite repression.

Phosphate inhibition of secondary metabolism in Streptomyces hygroscopicus and its reversal by cyclic AMP

Inorganic phosphate inhibited the biosynthesis of the macrolide antibiotic turimycin in different strains of Streptomyces hygroscopicus. In the wild type strain a depression was observed with increasing phosphate concentrations. A total inhibition was found at 0.1 M phosphate. In a high producing mutant a minimum of turimycin production occured when the phosphate concentration was between 5 mM and 10 mM. Above this concentration the antibiotic synthesis increased again but the production period shifted to a later period of cultivation. Addition of inorganic phosphate resulted in an initial increase of intracellular cyclic AMP content. But a second elevation characterizing the normal level of cyclic AMP throughout the growth phase was prevented by phosphate. Exogenous cyclic AMP as well as positive effectors of the adenylyl cyclase system were able to overcome the phosphate suppression. Cyclic AMP abolished the reduction of protein synthesis following phosphate addition and caused the reappearance of a protein band which may be responsible for the turimycin biosynthesis.