With four individual germ tubes (GTs), the distinct time of birth of branches was correlated to the kinetics of GT-elongation. During the first three hours, corresponding to five doubling times, branching was suppressed. The time of birth of the first branch (tbl) coincided with the 51h step of the typical stepwise increase of the GT elongation rate (GI). During this time, the quasi-exponential kinetics of a changed to linear kinetics, which was attained either at the 51h of 61h a-step. At the 51h step, the quotient of successive U-values had been reduced from 2 at the beginning of outgrowth to 1.2.Once the first branch had been formed, further branches appeared as clusters (additional 2, 4, and 8 branches) at D-intervals representing the doubling time of the hyphal length.The first branch appeard always in the vicinity of the spore. The average distance to the spore of 6.6 pm corresponded to four unit cells. At t b l , the unbranched region located between the first branch and the tip of the GTs was 42 pm long. The data concerning timing and location of the first branching in GTs suggest that initiation of branching was dependent on the accumulation of envelope precursors.Hitherto it has not been known how the formation of branches in mycelia of streptomycetes is regulated. Studies on the frequency of branching of mycelia grown in liquid media revealed that it was dependent on the growth rate, as well as on the composition of the media (RIESENBERG and BERGTER 1979, KRETSCHMER 1982). For example, in chemostats, with increasing growth rate the frequency of branching increased in the case of carbon limitation, but decreased, when nitrogen was limiting (KRETSCHMER 1985). This fact indicates that the formation of branches depends on the metabolism.However, this dependency may exist only indirectly. Morphogentic studies of the highly variable procaryotes Agrobacterium tumefactiens and Oerskovia xanthineolytica indicated that branches were initiated when the elongation rate (a) of the cell and hypha, respectively, approached its maximum value, while a, , , itself was dependent on the growth conditions (FUJIWARA andFUKUI 1974, KRETSCHMER 1981).In streptomycetes, the mode of increase of a in growing hyphae occurs stepwise until amax is attained (KRETSCHMER 1988). Using germ tubes of these organisms, the question of whether and how the mode of branching is correlated to the elongation behaviour can readily be studied.In the present paper, the timing and location of the first branching in germ tubes was correlated to their elengation behaviour by analyzing series of micrographs taken from individual germ tubes growing on agar medium.
Materials and methodsThe organism used was Streptomyces granaticolor strain ETH 7437. Spores were harvested from 8 days old slant cultures grown on oat meal agar and washed twice with distilled water. Methods used for microcultivation and phase contrast microscopy were as described before (KRETSCHMER 1978). The medium used for microcultivation was glucose-pepton agar (KRETSCHMER 1982) enriched...