Molecular cloning and characterization of the obg gene of Streptomyces griseus in relation to the onset of morphological differentiation

Abstract

Morphological differentiation in microorganisms is usually accompanied by a decrease in intracellular GTP pool size, as has been demonstrated in bacillaceae, streptomycetaceae, and yeasts. The obg gene, which codes for a GTP-binding protein belonging to the GTPase superfamily of proteins, was cloned from Streptomyces griseus IFO13189. The gene is located just downstream of the genes for ribosomal proteins L21 and L27, encoded a protein of 478 amino acids (51 kDa), and possessed three consensus motifs which confer GTPbinding ability; Obg protein expressed in Escherichia coli bound GTP, as demonstrated using a UV crosslinking method. Introduction of multiple copies of obg into wild-type S. griseus suppressed aerial mycelium development in cells on solid media. However, no effect on streptomycin production was detected, indicating that Obg is involved in the regulation of the onset of morphological but not physiological differentiation. Multiple copies of obg also suppressed submerged spore formation in liquid culture. Southern hybridization studies indicated that genes homologous to obg exist widely in streptomycetes, and an obg homolog was successfully cloned from S. coelicolor A3(2). We propose that by monitoring the intracellular GTP pool size, the Obg protein is involved in sensing changes in the nutritional environment leading ultimately to morphological differentiation.During evolution, organisms have learned to respond to certain environmental changes by producing specialized cell types. The signal initiating this development must persist for some time before the cells commit themselves irreversibly to differentiate. Several genera of bacteria, including Bacillus, Clostridium, Sporosarcina, Myxococcus, and Streptomyces, produce spores when they encounter adverse environmental conditions such as nutrient deficiency (reviewed by Freese and Heinze [14]). A distinguishing characteristic of species of the genus Streptomyces is their ability to form an aerial mycelium from a substrate mycelium when cultured on solid media, eventually leading to the formation of spores by the synchronous and regularly spaced septation of the aerial mycelium (for reviews, see references 5, 6, 7, and 18). Only a limited number of Streptomyces species such as S. griseus can produce spores (submerged spores) when cultured in liquid medium (26).Our laboratory (34,35,39) and the laboratory of Freese (15,30,40) have demonstrated that a decrease in GTP pool size correlates with the initiation of morphological differentiation in Bacillus subtilis and Streptomyces spp. Thus, these organisms can be induced to differentiate in nutritionally rich media, in which cells normally do not sporulate, (i) when the GTP pool size is reduced by the addition of decoyinine, an inhibitor of GMP synthesis; (ii) by the depletion of guanine in guaninerequiring auxotrophic mutants; or (iii) by provoking the stringent response in which ppGpp, a potent inhibitor of IMP dehydrogenase, accumulates. Although the role of GTP pool size variations is at present uncle...

Expression analysis of the ssgA gene product, associated with sporulation and cell division in Streptomyces griseus

“…The recent discovery of a GTP-binding protein, Obg, in S. griseus (Okamoto et al, 1997) provides further support for the role of GTP as a signal molecule in triggering morphological differentiation.…”

Guanine Nucleotides Guanosine 5′-Diphosphate 3′-Diphosphate and GTP Co-operatively Regulate the Production of an Antibiotic Bacilysin in Bacillus subtilis

“…In Streptomyces spp. the GTP-binding protein Obg was presented as a candidate protein for detecting the level of GTP for initiating morphological development of this genus (44,45). Introduction of multiple copies of obg into wild-type S. coelicolor suppresses not only aerial mycerium but also antibiotic (actinorhodin) production (45).…”

Nevel Function of GTP-Binding Protein and Ribosome in Initiating Cell Differentiation in Streptomyces.

“…The S. coelicolor obg gene was identified by Southern hybridization using S. griseus obg gene as a probe 11) . The S. coelicolor obg gene located between proA and clpP,X on its chromosome (on cosmid C88) and encoded a protein of 478 amino acids (51 kDa) with 85.7% sequence identity to the S. griseus Obg protein 12) .…”