Developmental Regulation of theStreptomyces lividans ramGenes: Involvement of RamR in Regulation of theramCSABOperon

Abstract

Streptomycetes are filamentous soil bacteria that produce spores through a complex process of morphological differentiation. The ram cluster plays an important part during the development. The ram genes encode a membrane-bound kinase (RamC), a small protein (RamS), components of an ABC transporter (RamAB), and a response regulator (RamR). While the introduction of an extra copy of the ram cluster accelerates development in Streptomyces lividans, ramABR disruption mutants are unable to produce aerial hyphae and spores. The developmental regulation of ram gene transcription was analyzed. Transcription of the ram genes occurred only on solid rich media and not on minimal media. The ramR gene is transcribed from a single promoter during all growth stages, with the highest levels during aerial growth. The ramCSAB genes comprise one operon and are transcribed from one principal promoter, P1, directly upstream of ramC. Transcription of ramCSAB was already observed during vegetative growth, but was strongly upregulated upon initiation of formation of aerial hyphae and was decreased during late stages of development. A large inverted repeat located downstream of ramS terminated the majority of transcripts. The introduction of ramR on a multicopy vector in S. lividans strongly induced P1 activity, while disruption of this regulator eliminated all P1 promoter activity. This shows that ramR is a crucial activator of ramCSAB transcription. Importantly, in bldA, bldB, bldD, or bldH mutants, ramR and ramCSAB are not transcribed, while ram gene transcription was observed in the earliest whi mutant, whiG. This indicates that the transcription of the ram genes marks the transition from vegetative to aerial growth.Many unicellular bacteria can temporarily adopt a multicellular organization to perform specific tasks. Examples of this include swarming, in which numerous cells aggregate and move collectively over a surface, and myxospore formation, which is preceded by aggregation resulting in the formation of a sacculus (stalk of fruiting body) allowing the subsequent development of resistant cells (myxospores) (31). Different from most prokaryotes, the gram-positive soil bacterium Streptomyces mainly exists as a multicellular organism. Streptomycetes rely on a process of spore formation for propagation and survival. The life cycle of these organisms resembles that of filamentous fungi. After germination of a spore, a network of branched filamentous vegetative hyphae is produced. At late vegetative growth, secondary metabolism is switched on and reproductive growth is initiated. One of the first steps in reproductive growth is the production of hydrophobic aerial hyphae that erect themselves out of the plane of the agar. The initiation of formation of aerial hyphae is believed to be dependent on metabolic status, stress, and a cascade of extracellular signaling molecules (17, 41). Aerial hyphae undergo several transformation steps ultimately leading to the formation of unigenomic spores at the hyphal tips (5, 9). Elemental steps in ...

Dual Transcriptional Control of amfTSBA , Which Regulates the Onset of Cellular Differentiation in Streptomyces griseus

“…The dots represent sequence gaps between the two orthologous regions. The transcriptional start site of ramC has not been identified in S. coelicolor A3 (2), and the site presented is that determined in S. lividans by Keiser et al (12). The nucleotide sequences of the ramC promoter regions of S. coelicolor and S. lividans are identical.…”

“…Recently, extensive studies by three research teams have shown that the ram gene cluster plays a significant regulatory role in developmental regulation in S. coelicolor A3(2), the best-studied model organism (12,20). They revealed that promoter activities for ramR and ramCSAB, equivalent to amfR and amfTSBA, respectively, are developmentally regulated, and inactivation of either ramR or ramC abolishes aerial growth.…”

ThebldCDevelopmental Locus ofStreptomyces coelicolorEncodes a Member of a Family of Small DNA-Binding Proteins Related to the DNA-Binding Domains of the MerR Family

“…Expression of the chaplin genes is developmentally regulated, and their transcription is blocked in all the bld mutants tested, including bldC (19). Expression of the ram genes is also developmentally regulated, and ramR and ramCSAB transcription is blocked in bldA, bldB, bldD, and bldH mutants (bldC has not been tested) (25). Further, almost all bld mutants regain the ability to form aerial structures when purified SapB is applied to the colony surface (51).…”

“…Conversely, overexpression of ramR results in SapB overproduction and the biosynthesis of SapB by wild-type strains under conditions when its production is normally repressed (38). The ram genes are not transcribed during growth on minimal medium (25).…”

“…Aerial mycelium formation on minimal medium is SapB independent, and SapB is not produced (51). The ram cluster consists of five genes, the SapB biosynthetic operon itself (ramCSAB) and the divergently encoded response regulator, RamR, which activates transcription of the ramCSAB operon on rich medium (25,38,40). SapB is derived from the 42-amino-acid primary translation product of the ramS gene through extensive posttranslational modification (29).…”