AN ESSENTIAL GTP-BINDING PROTEIN FUNCTIONS AS A REGULATOR FOR DIFFERENTIATION IN STREPTOMYCES-COELICOLOR

The Streptomyces coelicolor obg gene, which encodes a putative GTP-bin ding protein of the Obg/Gtp1 family, was characterized. The obg gene w as essential for viability. Introduction of multiple copies of obg int o wild-type S. coelicolor suppressed aerial mycelium formation. A sing le amino acid substitution at any of six positions was introduced into the GTP binding site of Obg, and the mutated proteins were expressed in wild-type cells. Obg(P168-->V) exerted a more accentuated suppressi ve effect on aerial mycelium formation than did the wild-type Obg prot ein. In contrast, Obg(G171-->A) accelerated the development of aerial mycelium. These results show that Obg protein functions as a pivotal r egulator for the onset of cell differentiation through its ability to bind GTP. Western analysis revealed that expression of obg is regulate d in a growth phase-dependent manner, indicating a sharp decrease just after onset of aerial mycelium development or at the end of vegetativ e growth. Obg was a membrane-bound protein as determined by immunoelec tron microscopy.