Cloning of Genes in Streptomycetes for Secondary Metabolism

Summary

Streptomycetes are Gram-positive bacteria with an extremely high guanine plus cystosine content (70 to 73%). Their capability to produce the great majority of secondary metabolites including most of the antibiotics makes them industrially important. A complex process of morphological differentiation displayed by Streptomyces also has biologically interesting aspects. A close relationship of secondary metabolism with cell differentiation is well recognized in streptomycetes. It seems reasonable to assume that multiple genes involved in both the complex processes are controlled by a common regulatory gene or substance.

A representative of such regulatory substances is A-factor (2-isocaρryloyl-3R-hydroxymethyl-γ-butyrolactone; Mori, 1983) which was originally found by Khokhlov et al. (1967) in the culture broth of Streptomyces griseus (Figure 1). A-factor is a self-regulatory substance or bioregulator essential for streptomycin production, streptomycin resistance, and spore formation in this organism (Khokhlov et al., 1973; Khoklov, 1980; Hara and Beppu, 1982a, 1982b). A-factor-deficient mutants of S. griseus simultaneously lose streptomycin production and resistance, and spore forming ability. In such mutants, addition of A-factor at a concentration of 10^-9 M restores all the defects. These features of A-factor are similar to hormones in eukaryotes.

Recent development of host-vector systems for streptomycetes has enabled us to clone various genes involved in secondary metabolism. In order to clarify the genetic background of the A-factor regulatory system, we have cloned and characterized an A-factor determinant as well as a streptomycin synthesizing gene as a possible target of the A-factor function

from streptomycin-producing Streptomyces bikiniensis. In the course of the experiments, we also found that a regulatory gene, afsB, positively controlling biosynthesis of A-factor in Streptomyces coelicolor A3(2) simultaneously regulated biosynthesis of pigmented antibiotics actinorhodin and undecylprodigiosin in this organism as well as in Streptomyces lividans. We will describe below cloning and characterization of these genes involved in secondary metabolism.