Autoregulatory factors and communication in actinomycetes

The ability to produce a wide variety of secondary metabolites and a mycelial form of growth that develops into spores are two biological aspects characteristic of the gram-positive bacterial genus Streptomyces. Secondary metabolism and cell differentiation are controlled by diffusible low-molecular-weight chemical substances called autoregulators. A-factor, the representative of the autoregulators, triggers streptomycin production and aerial-mycelium formation in Streptomyces griseus. A-factor exerts its regulatory function with the aid of a receptor protein that itself acts as a repressor-type regulator. The A-factor signal via the A-factor-receptor protein is transferred to downstream genes, such as streptomycin-production genes and sporulation genes, through multiple regulatory genes in a complex regulatory cascade. Thus, A-factor can be termed a "microbial hormone." This review deals with the A-factor-regulatory cascade as a model system for other autoregulators. The biosynthesis of A-factor, the structures and characteristics of other autoregulators, and the importance of these autoregulators in the ecosystem are also included.