Paradoxical reaction ofStreptomyces avermitilis to ultraviolet irradiation as a criterion of the appearance of highly active variants in a culture

Abstract

The paradoxical reaction of Streptomyces avermitilis BKM Ac 1301 to ultraviolet light (UV) is revealed. The reaction manifests itself as a drop of lethal and mutagenic activities after exposure to increased dose of UV. Simultaneous changes in the survival of spores and in the number of morphological mutations associated with this reaction are proposed as a criterion for selecting effective doses of mutagen. Key Words: streptomycetes; mutagenesis; UV rays; criterionAvermectin-based preparations effectively control animal, plant, and human parasites. The nematocidal, acaricidal, and insecticidal activities of avermectins are due to blockade of synaptic transmission in invertebrates. The avermectin complex is produced by Streptomyces avermitilis [6]. Highly productive commercial strains are obtained by individual selection of strain and mutagen. We studied the specific reaction of Streptomyces avermitilis to ultraviolet (UV) exposure.It was expected that the regularities revealed would be helpful for the choice of effective doses and for the development of highly productive strains. with UV (dose range 1000-9000 erg/mm 2) by the conventional method [3]. Spores were kept at a low temperature for 7 days, seeded in Petri dishes with 2% glucose-potato agar, and incubated at 28~ in a thermostat. The lethal effect was assessed by counting the spores surviving after the exposure. Morphologically changed variants (MCV), biochemical mutants (auxotrophs), and changes in biosynthetic activity were assessed. Auxotrophs were detected by total sifting [2] on minimal medium with glucose. Biosynthetic activity (the capacity of a culture to produce avermectin) was assessed by accumulation of avermectins in the mycelium [4]. MATERIALS AND METHODS Streptomyces avermitilis RESULTSFigures 1 and 2 illustrate the relationships between the studied parameters of Streptomyces avermitilis culture (survival of spores, number of MCV, and activity of avermectin production) and UV dose. All these curves can be divided into two portions reflecting different patterns of changes; the critical