Actinobacteria are Gram-positive bacteria that are ubiquitous and abundant in soil. They develop vegetative hyphae, aerial mycelium and conidial spores during their life cycle, thus being morphologically resembling fungi. The interesting and important property of Streptomycetes bacteria is their ability of producing a variety of antibiotics such as Streptomycin, Erythromycin, Tetracycline etc. through complex secondary metabolic pathways. The antibiotics produced by them account for 60% of naturally-occurring antibiotics and are used as antibacterial, antifungal, antiviral, anitiparasitic, immunosuppressant and antitumor medicines.

Streptomyces avermitilis (or, Streptomyces avermectinius) MA-4680^T is the producer of anthelmintic macrolide "avermectin" that was isolated by Omura et al. of the Kitasato Institute from the soil sample collected in Ito City, Shizuoka Pref. It has an unusually large genome of 9.02 Mb of high G+C content, which, similar to the cases of other bacteria belonging to the genus Streptomyces, exists as a linear chromosome. Both ends (telomeres) of a linear chromosome contain terminal inverted repeats and covalently binding terminal proteins (TPs). Its genome analysis led to the identification of 30 gene clusters involved in secondary metabolite biosynthesis, a half of which is located near either end of the chromosome. Of them, twelve were found to be involved in polyketide synthesis. Analysis of these gene clusters along with enzymes involved would reveal clues as to the synthesis of many secondary metabolites and their physiological roles.