Abstract
Streptomycetes are filamentous bacteria that differentiate by producing spore-bearing reproductive structures called aerial hyphae. The transition from vegetative to reproductive growth is controlled by the bld (bald) loci, and mutations in bld genes prevent the formation of aerial hyphae, either by blocking entry into development (typically mutations in activators) or by inducing precocious sporulation in the vegetative mycelium (typically mutations in repressors). One of the bld genes, bldC, encodes a 68-residue DNA-binding protein related to the DNA-binding domain of MerR-family transcription factors. Recent work has shown that BldC binds DNA by a novel mechanism, but there is less insight into its impact on Streptomyces development. Here we used ChIP-seq coupled with RNA-seq to define the BldC regulon in the model species Streptomyces venezuelae, showing that BldC can function both as a repressor and as an activator of transcription. Using electron microscopy and time-lapse imaging, we show that bldC mutants are bald because they initiate development prematurely, bypassing the formation of aerial hyphae. This is consistent with the premature expression of BldC target genes encoding proteins with key roles in development (e.g., whiD, whiI, sigF), chromosome condensation and segregation (e.g., smeA-sffA, hupS), and sporulation-specific cell division (e.g., dynAB), suggesting that BldC-mediated repression is critical to maintain a sustained period of vegetative growth prior to sporulation. We discuss the possible significance of BldC as an evolutionary link between MerR family transcription factors and DNA architectural proteins.IMPORTANCE Understanding the mechanisms that drive bacterial morphogenesis depends on the dissection of the regulatory networks that underpin the cell biological processes involved. Recently, Streptomyces venezuelae has emerged as an attractive model system for the study of morphological differentiation in Streptomyces This has led to significant progress in identifying the genes controlled by the transcription factors that regulate aerial mycelium formation (Bld regulators) and sporulation (Whi regulators). Taking advantage of S. venezuelae, we used ChIP-seq coupled with RNA-seq to identify the genes directly under the control of BldC. Because S. venezuelae sporulates in liquid culture, the complete spore-to-spore life cycle can be examined using time-lapse microscopy, and we applied this technique to the bldC mutant. These combined approaches reveal BldC to be a member of an emerging class of Bld regulators that function principally to repress key sporulation genes, thereby extending vegetative growth and blocking the onset of morphological differentiation.
Highlights
Streptomycetes are filamentous bacteria that differentiate by producing spore-bearing reproductive structures called aerial hyphae
We show that bldC mutants are bald because they enter development prematurely, bypassing the formation of aerial hyphae
After 3 days, the aerial hyphae of the wild type had differentiated into spores, and essentially the entire biomass of the bldC mutant had differentiated into spores, bypassing aerial mycelium formation
Summary
Streptomycetes are filamentous bacteria that differentiate by producing spore-bearing reproductive structures called aerial hyphae. Because S. venezuelae sporulates in liquid culture, the complete spore-to-spore life cycle can be examined using time-lapse microscopy, and we applied this technique to the bldC mutant These combined approaches reveal BldC to be a member of an emerging class of Bld regulators that function principally to repress key sporulation genes, thereby extending vegetative growth and blocking the onset of morphological differentiation. A smaller but growing class of bld mutants, which define negative regulators, enter development prematurely, inducing sporulation in the vegetative mycelium and bypassing the formation of aerial hyphae Macroscopically these two classes of mutants look similar, forming smooth colonies that lack the “hairy” appearance of the wild type, but microscopically it is apparent that they arise for diametrically opposed reasons [5, 7,8,9]. MerR-like proteins share significant sequence similarity only within their DNA-binding domains; as different family members bind different effectors, their C-terminal domains are variable and show little, if any, similarity to one another
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