Dependence of macromolecular composition and morphology of Streptomyces hygroscopicus on specific growth rate

The dependence of macromolecular composition and morphology of Streptomyces hygroscopicus on specific growth rate μ was investigated. The percentage of DNA on dry weight (%DNA) is constant, %protein is also nearly independent of μ whereas %RNA rises considerably with increasing μ, regarding mycelia grown in glucose‐limited and ammonium‐limited continuous cultures as well as in discontinuous cultures with various carbon sources. It is probable that the overall synthesis of DNA, RNA and protein is regulated in the mycelium‐forming bacterium S. hygroscopicus by the same mechanisms found in unicellular bacteria like Escherichia coli because of the qualitatively similar dependence of %DNA, %RNA and %protein on μ. But differences exist in quantitative regard whereby %DNA, %RNA and %protein of S. hygroscopicus are much smaller at low μ and, with increasing μ, approach those of unicellular bacteria. The hypothesis about the increase of the hyphal regions showing high synthesis activity in S. hygroscopicus mycelia grown in glucose‐limited continuous cultures with increasing μ – derived from comparison of macromolecular composition of S. hygroscopicus and unicellular bacteria – was confirmed autoradiographically with respect to protein synthesis. The increase of the part of mycelial regions showing high cytoplasmic activity results in an increase of mean hyphal diameter, of mean relative apical growth rate α and/ or mean relative branching rate ß.ß depends sigmoidally and α inverses sigmoidally on μ. Therefore, the morphology of the mycelium determined by α and also depends on μ. The hyphal growth unit L/N, the distance from apex to first branch Lp and the mean distance between neighbouring branches Ln decline with increasing μ and reach a minimum at μ = 0.32 (1/h). A further rise of μ is accompanied with an increase of L/N, Lp and Ln. This means that mycelia growing slowly or very quickly have a loose from whereas quickly growing mycelia are characterized by a more compact form. The complicated dependence of α, β, L/N, Lp and Ln on μ indicates tha the morphology is regulated by different mechanisms depending on the specific growth rate.