The streptomycin (Sm) producer, N2-3-11, shows medium-independent biphasic kinetics of the vegetative or exponential growth phase (EGP), reflecting an innate clock-like behaviour of growth and differentiation. The growth and development cycle has the following characteristics: (1) after the developmental cycle commences, it cannot be influenced by environmental conditions; (2) the first EGP (decision phase) and its duration seem to be genetically determined, and it is also exhibited in pleiotropic mutants deficient in differentiation and antibiotic production; (3) during this early phase of growth, the decision to produce Sm is established and the fixation of later production and differentiation can only be influenced by effector molecules, e.g. A-factor, during this period; (4) after the onset of the second EGP, the commitment to Sm production cannot be reversed by dilution into fresh medium, nor by effector molecules; (5) the length of time of this effector-insensitive growth phase (second EGP or execution phase) can be extended by dilution into fresh medium; (6) the differentiation cycle of is completed on entering stationary phase. The cells of then return to a decision-making stage and recover sensitivity to effector molecules. Evidence that this type of phasing is valid for the growth and developmental cycles of all streptomycetes is discussed.


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