Tetrameric c-di-GMP mediates effective transcription factor dimerization to control Streptomyces development.

The cyclic dinucleotide c-di-GMP is a signaling molecule with diverse functions in cellular physiology. Here, we report that c-di-GMP can assemble into a tetramer that mediates the effective dimerization of a transcription factor, BldD, which controls the progression of multicellular differentiation in sporulating actinomycete bacteria. BldD represses expression of sporulation genes during vegetative growth in a manner that depends on c-di-GMP-mediated dimerization. Structural and biochemical analyses show that tetrameric c-di-GMP links two subunits of BldD through their C-terminal domains, which are otherwise separated by ~10 Å and thus cannot effect dimerization directly. Binding of the c-di-GMP tetramer by BldD is selective and requires a bipartite RXD-X8-RXXD signature. The findings indicate a unique mechanism of protein dimerization and the ability of nucleotide signaling molecules to assume alternative oligomeric states to effect different functions.

c-di-GMP Arms an Anti-σ to Control Progression of Multicellular Differentiation in Streptomyces.

Streptomyces are our primary source of antibiotics, produced concomitantly with the transition from vegetative growth to sporulation in a complex developmental life cycle. We previously showed that the signaling molecule c-di-GMP binds BldD, a master repressor, to control initiation of development. Here we demonstrate that c-di-GMP also intervenes later in development to control differentiation of the reproductive hyphae into spores by arming a novel anti-σ (RsiG) to bind and sequester a sporulation-specific σ factor (σWhiG). We present the structure of the RsiG-(c-di-GMP)2-σWhiG complex, revealing an unusual, partially intercalated c-di-GMP dimer bound at the RsiG-σWhiG interface. RsiG binds c-di-GMP in the absence of σWhiG, employing a novel E(X)3S(X)2R(X)3Q(X)3D motif repeated on each helix of a coiled coil. Further studies demonstrate that c-di-GMP is essential for RsiG to inhibit σWhiG. These findings reveal a newly described control mechanism for σ-anti-σ complex formation and establish c-di-GMP as the central integrator of Streptomyces development.

Structural basis of dual activation of cell division by the actinobacterial transcription factors WhiA and WhiB.

Studies of transcriptional initiation in different bacterial clades reveal diverse molecular mechanisms regulating this first step in gene expression. The WhiA and WhiB factors are both required to express cell division genes in Actinobacteria and are essential in notable pathogens such as Mycobacterium tuberculosis. The WhiA/B regulons and binding sites have been elucidated in Streptomyces venezuelae (Sven), where they coordinate to activate sporulation septation. However, how these factors cooperate at the molecular level is not understood. Here we present cryoelectron microscopy structures of Sven transcriptional regulatory complexes comprising RNA polymerase (RNAP) σA-holoenzyme and WhiA and WhiB, in complex with the WhiA/B target promoter sepX. These structures reveal that WhiB binds to domain 4 of σA (σA4) of the σA-holoenzyme, bridging an interaction with WhiA while making non-specific contacts with the DNA upstream of the -35 core promoter element. The N-terminal homing endonuclease-like domain of WhiA interacts with WhiB, while the WhiA C-terminal domain (WhiA-CTD) makes base-specific contacts with the conserved WhiA GACAC motif. Notably, the structure of the WhiA-CTD and its interactions with the WhiA motif are strikingly similar to those observed between σA4 housekeeping σ-factors and the -35 promoter element, suggesting an evolutionary relationship. Structure-guided mutagenesis designed to disrupt these protein-DNA interactions reduces or abolishes developmental cell division in Sven, confirming their significance. Finally, we compare the architecture of the WhiA/B σA-holoenzyme promoter complex with the unrelated but model CAP Class I and Class II complexes, showing that WhiA/WhiB represent a new mechanism in bacterial transcriptional activation.

Evolution of a σ-(c-di-GMP)-anti-σ switch.

Filamentous actinobacteria of the genus Streptomyces have a complex lifecycle involving the differentiation of reproductive aerial hyphae into spores. We recently showed c-di-GMP controls this transition by arming a unique anti-σ, RsiG, to bind the sporulation-specific σ, WhiG. The Streptomyces venezuelae RsiG-(c-di-GMP)2-WhiG structure revealed that a monomeric RsiG binds c-di-GMP via two E(X)3S(X)2R(X)3Q(X)3D repeat motifs, one on each helix of an antiparallel coiled-coil. Here we show that RsiG homologs are found scattered throughout the Actinobacteria. Strikingly, RsiGs from unicellular bacteria descending from the most basal branch of the Actinobacteria are small proteins containing only one c-di-GMP binding motif, yet still bind their WhiG partners. Our structure of a Rubrobacter radiotolerans (RsiG)2-(c-di-GMP)2-WhiG complex revealed that these single-motif RsiGs are able to form an antiparallel coiled-coil through homodimerization, thereby allowing them to bind c-di-GMP similar to the monomeric twin-motif RsiGs. Further data show that in the unicellular actinobacterium R. radiotolerans, the (RsiG)2-(c-di-GMP)2-WhiG regulatory switch controls type IV pilus expression. Phylogenetic analysis indicates the single-motif RsiGs likely represent the ancestral state and an internal gene-duplication event gave rise to the twin-motif RsiGs inherited elsewhere in the Actinobacteria. Thus, these studies show how the anti-σ RsiG has evolved through an intragenic duplication event from a small protein carrying a single c-di-GMP binding motif, which functions as a homodimer, to a larger protein carrying two c-di-GMP binding motifs, which functions as a monomer. Consistent with this, our structures reveal potential selective advantages of the monomeric twin-motif anti-σ factors.

Global Effects of the Developmental Regulator BldB in Streptomyces venezuelae.

In Streptomyces, the Bld (Bald) regulators control formation of the reproductive aerial hyphae. The functions of some of these regulators have been well characterized, but BldB has remained enigmatic. In addition to the bldB gene itself, Streptomyces venezuelae has 10 paralogs of bldB that sit next to paralogs of whiJ and abaA. Transcriptome sequencing (RNA-seq) revealed that loss of BldB function causes the dramatic transcriptional upregulation of the abaA paralogs and a novel inhibitor of sporulation, iosA, and that cooverexpression of just two of these genes, iosA and abaA6, was sufficient to recapitulate the bldB mutant phenotype. Further RNA-seq analysis showed that the transcription factor WhiJ9 is required for the activation of iosA seen in the bldB mutant, and biochemical studies showed that WhiJ9 mediates the activation of iosA expression by binding to direct repeats in the iosA-whiJ9 intergenic region. BldB and BldB9 hetero-oligomerize, providing a potential link between BldB and the iosA-whiJ9-bldB9 locus. This work greatly expands our overall understanding of the global effects of the BldB developmental regulator. IMPORTANCE To reproduce and disperse, the filamentous bacterium Streptomyces develops specialized reproductive structures called aerial hyphae. The formation of these structures is controlled by the bld (bald) genes, many of which encode transcription factors whose functions have been characterized. An exception is BldB, a protein whose biochemical function is unknown. In this study, we gain insight into the global effects of BldB function by examining the genome-wide transcriptional effects of deleting bldB. We identify a small set of genes that are dramatically upregulated in the absence of BldB. We show that their overexpression causes the bldB phenotype and characterize a transcription factor that mediates the upregulation of one of these target genes. Our results provide new insight into how BldB influences Streptomyces development.

Expansion and re-classification of the extracytoplasmic function (ECF) σ factor family.

Extracytoplasmic function σ factors (ECFs) represent one of the major bacterial signal transduction mechanisms in terms of abundance, diversity and importance, particularly in mediating stress responses. Here, we performed a comprehensive phylogenetic analysis of this protein family by scrutinizing all proteins in the NCBI database. As a result, we identified an average of ∼10 ECFs per bacterial genome and 157 phylogenetic ECF groups that feature a conserved genetic neighborhood and a similar regulation mechanism. Our analysis expands previous classification efforts ∼50-fold, enriches many original ECF groups with previously unclassified proteins and identifies 22 entirely new ECF groups. The ECF groups are hierarchically related to each other and are further composed of subgroups with closely related sequences. This two-tiered classification allows for the accurate prediction of common promoter motifs and the inference of putative regulatory mechanisms across subgroups composing an ECF group. This comprehensive, high-resolution description of the phylogenetic distribution of the ECF family, together with the massive expansion of classified ECF sequences and an openly accessible data repository called 'ECF Hub' (https://www.computational.bio.uni-giessen.de/ecfhub), will serve as a powerful hypothesis-generator to guide future research in the field.

Interaction of the Streptomyces Wbl protein WhiD with the principal sigma factor σHrdB depends on the WhiD [4Fe-4S] cluster.

The bacterial protein WhiD belongs to the Wbl family of iron-sulfur [Fe-S] proteins present only in the actinomycetes. In Streptomyces coelicolor, it is required for the late stages of sporulation, but precisely how it functions is unknown. Here, we report results from in vitro and in vivo experiments with WhiD from Streptomyces venezuelae (SvWhiD), which differs from S. coelicolor WhiD (ScWhiD) only at the C terminus. We observed that, like ScWhiD and other Wbl proteins, SvWhiD binds a [4Fe-4S] cluster that is moderately sensitive to O2 and highly sensitive to nitric oxide (NO). However, although all previous studies have reported that Wbl proteins are monomers, we found that SvWhiD exists in a monomer-dimer equilibrium associated with its unusual C-terminal extension. Several Wbl proteins of Mycobacterium tuberculosis are known to interact with its principal sigma factor SigA. Using bacterial two-hybrid, gel filtration, and MS analyses, we demonstrate that SvWhiD interacts with domain 4 of the principal sigma factor of Streptomyces, σHrdB (σHrdB4). Using MS, we determined the dissociation constant (Kd ) for the SvWhiD-σHrdB4 complex as ∼0.7 µm, consistent with a relatively tight binding interaction. We found that complex formation was cluster dependent and that a reaction with NO, which was complete at 8-10 NO molecules per cluster, resulted in dissociation into the separate proteins. The SvWhiD [4Fe-4S] cluster was significantly less sensitive to reaction with O2 and NO when SvWhiD was bound to σHrdB4, consistent with protection of the cluster in the complex.

Streptomyces venezuelae NRRL B-65442: genome sequence of a model strain used to study morphological differentiation in filamentous actinobacteria.

For over a decade, Streptomyces venezuelae has been used to study the molecular mechanisms that control morphological development in streptomycetes and is now a well-established model strain. Its rapid growth and ability to sporulate in a near-synchronised manner in liquid culture, unusual among streptomycetes, greatly facilitates the application of modern molecular techniques such as ChIP-seq and RNA-seq, as well as time-lapse fluorescence imaging of the complete Streptomyces life cycle. Here we describe a high-quality genome sequence of our isolate of the strain (Northern Regional Research Laboratory [NRRL] B-65442) consisting of an 8.2 Mb chromosome and a 158 kb plasmid, pSVJI1, which had not been reported previously. Surprisingly, while NRRL B-65442 yields green spores on MYM agar, the American Type Culture Collection (ATCC) type strain 10712 (from which NRRL B-65442 was derived) produces grey spores. While comparison of the genome sequences of the two isolates revealed almost total identity, it did reveal a single nucleotide substitution in a gene, vnz_33525, involved in spore pigment biosynthesis. Replacement of the vnz_33525 allele of ATCC 10712 with that of NRRL B-65442 resulted in green spores, explaining the discrepancy in spore pigmentation. We also applied CRISPR-Cas9 to delete the essential parB of pSVJI1 to cure the plasmid from the strain without obvious phenotypic consequences.

Discovery of the extracytoplasmic function σ factors.

This special issue of Molecular Microbiology marks the 25th anniversary of the discovery of the extracytoplasmic function (ECF) σ factors, proteins that subsequently emerged as the largest group of alternative σ factors and one of the three major pillars of signal transduction in bacteria, alongside one- and two-component systems. A single bacterial genome can encode > 100 ECF σ factors, and combined with their cognate anti-σ factors, they represent a modular design that primarily functions in transmembrane signal transduction. Here, we first describe the immediate events that led to the 1994 publication in the Proceeding of the National Academy of Sciences USA, and then set them in the broader context of key events in the history of σ biology research.

Defining the regulon of genes controlled by σE , a key regulator of the cell envelope stress response in Streptomyces coelicolor.

The extracytoplasmic function (ECF) σ factor, σE , is a key regulator of the cell envelope stress response in Streptomyces coelicolor. Although its role in maintaining cell wall integrity has been known for over a decade, a comprehensive analysis of the genes under its control has not been undertaken. Here, using a combination of chromatin immunoprecipitation-sequencing (ChIP-seq), microarray transcriptional profiling and bioinformatic analysis, we attempt to define the σE regulon. Approximately half of the genes identified encode proteins implicated in cell envelope function. Seventeen novel targets were validated by S1 nuclease mapping or in vitro transcription, establishing a σE -binding consensus. Subsequently, we used bioinformatic analysis to look for conservation of the σE target promoters identified in S. coelicolor across 19 Streptomyces species. Key proteins under σE control across the genus include the actin homolog MreB, three penicillin-binding proteins, two L,D-transpeptidases, a LytR-CpsA-Psr-family protein predicted to be involved in cell wall teichoic acid deposition and a predicted MprF protein, which adds lysyl groups to phosphatidylglycerol to neutralize membrane surface charge. Taken together, these analyses provide biological insight into the σE -mediated cell envelope stress response in the genus Streptomyces.

The crystal structure of the RsbN-σBldN complex from Streptomyces venezuelae defines a new structural class of anti-σ factor.

Streptomyces are filamentous bacteria with a complex developmental life cycle characterized by the formation of spore-forming aerial hyphae. Transcription of the chaplin and rodlin genes, which are essential for aerial hyphae production, is directed by the extracytoplasmic function (ECF) σ factor BldN, which is in turn controlled by an anti-σ factor, RsbN. RsbN shows no sequence similarity to known anti-σ factors and binds and inhibits BldN in an unknown manner. Here we describe the 2.23 Å structure of the RsbN-BldN complex. The structure shows that BldN harbors σ2 and σ4 domains that are individually similar to other ECF σ domains, which bind -10 and -35 promoter regions, respectively. The anti-σ RsbN consists of three helices, with α3 forming a long helix embraced between BldN σ2 and σ4 while RsbN α1-α2 dock against σ4 in a manner that would block -35 DNA binding. RsbN binding also freezes BldN in a conformation inactive for simultaneous -10 and -35 promoter interaction and RNAP binding. Strikingly, RsbN is structurally distinct from previously solved anti-σ proteins. Thus, these data characterize the molecular determinants controlling a central Streptomyces developmental switch and reveal RsbN to be the founding member of a new structural class of anti-σ factor.

Two dynamin-like proteins stabilize FtsZ rings during Streptomyces sporulation.

During sporulation, the filamentous bacteria Streptomyces undergo a massive cell division event in which the synthesis of ladders of sporulation septa convert multigenomic hyphae into chains of unigenomic spores. This process requires cytokinetic Z-rings formed by the bacterial tubulin homolog FtsZ, and the stabilization of the newly formed Z-rings is crucial for completion of septum synthesis. Here we show that two dynamin-like proteins, DynA and DynB, play critical roles in this process. Dynamins are a family of large, multidomain GTPases involved in key cellular processes in eukaryotes, including vesicle trafficking and organelle division. Many bacterial genomes encode dynamin-like proteins, but the biological function of these proteins has remained largely enigmatic. Using a cell biological approach, we show that the two Streptomyces dynamins specifically localize to sporulation septa in an FtsZ-dependent manner. Moreover, dynamin mutants have a cell division defect due to the decreased stability of sporulation-specific Z-rings, as demonstrated by kymographs derived from time-lapse images of FtsZ ladder formation. This defect causes the premature disassembly of individual Z-rings, leading to the frequent abortion of septum synthesis, which in turn results in the production of long spore-like compartments with multiple chromosomes. Two-hybrid analysis revealed that the dynamins are part of the cell division machinery and that they mediate their effects on Z-ring stability during developmentally controlled cell division via a network of protein-protein interactions involving DynA, DynB, FtsZ, SepF, SepF2, and the FtsZ-positioning protein SsgB.

The Streptomyces master regulator BldD binds c-di-GMP sequentially to create a functional BldD2-(c-di-GMP)4 complex.

Streptomyces are ubiquitous soil bacteria that undergo a complex developmental transition coinciding with their production of antibiotics. This transition is controlled by binding of a novel tetrameric form of the second messenger, 3΄-5΄ cyclic diguanylic acid (c-di-GMP) to the master repressor, BldD. In all domains of life, nucleotide-based second messengers allow a rapid integration of external and internal signals into regulatory pathways that control cellular responses to changing conditions. c-di-GMP can assume alternative oligomeric states to effect different functions, binding to effector proteins as monomers, intercalated dimers or, uniquely in the case of BldD, as a tetramer. However, at physiological concentrations c-di-GMP is a monomer and little is known about how higher oligomeric complexes assemble on effector proteins and if intermediates in assembly pathways have regulatory significance. Here, we show that c-di-GMP binds BldD using an ordered, sequential mechanism and that BldD function necessitates the assembly of the BldD2-(c-di-GMP)4 complex.

Multi-layered inhibition of Streptomyces development: BldO is a dedicated repressor of whiB.

BldD-(c-di-GMP) sits on top of the regulatory network that controls differentiation in Streptomyces, repressing a large regulon of developmental genes when the bacteria are growing vegetatively. In this way, BldD functions as an inhibitor that blocks the initiation of sporulation. Here, we report the identification and characterisation of BldO, an additional developmental repressor that acts to sustain vegetative growth and prevent entry into sporulation. However, unlike the pleiotropic regulator BldD, we show that BldO functions as the dedicated repressor of a single key target gene, whiB, and that deletion of bldO or constitutive expression of whiB is sufficient to induce precocious hypersporulation.

Actinoplanes Swims into the Molecular Age.

The survival strategy of Actinoplanes is fascinating from an evolutionary perspective, combining a short motile phase in an otherwise nonmotile, filamentous life cycle and the somewhat paradoxical concept of spores-normally thought of as a resting stage-that swim. In the first paper to report a molecular genetic analysis of development in Actinoplanes, the authors identify a key regulator of the entry into development (Y. Mouri, K. Konishi, A. Fujita, T. Tezuka, Y. Ohnishi, J Bacteriol 199:e00840-16, 2017, https://doi.org/10.1128/JB.00840-16).

Response regulator heterodimer formation controls a key stage in Streptomyces development.

The orphan, atypical response regulators BldM and WhiI each play critical roles in Streptomyces differentiation. BldM is required for the formation of aerial hyphae, and WhiI is required for the differentiation of these reproductive structures into mature spores. To gain insight into BldM function, we defined the genome-wide BldM regulon using ChIP-Seq and transcriptional profiling. BldM target genes clustered into two groups based on their whi gene dependency. Expression of Group I genes depended on bldM but was independent of all the whi genes, and biochemical experiments showed that Group I promoters were controlled by a BldM homodimer. In contrast, Group II genes were expressed later than Group I genes and their expression depended not only on bldM but also on whiI and whiG (encoding the sigma factor that activates whiI). Additional ChIP-Seq analysis showed that BldM Group II genes were also direct targets of WhiI and that in vivo binding of WhiI to these promoters depended on BldM and vice versa. We go on to demonstrate that BldM and WhiI form a functional heterodimer that controls Group II promoters, serving to integrate signals from two distinct developmental pathways. The BldM-WhiI system thus exemplifies the potential of response regulator heterodimer formation as a mechanism to expand the signaling capabilities of bacterial cells.

A sporulation-specific, sigF-dependent protein, SspA, affects septum positioning in Streptomyces coelicolor.

The RNA polymerase sigma factor SigF controls late development during sporulation in the filamentous bacterium Streptomyces coelicolor. The only known SigF-dependent gene identified so far, SCO5321, is found in the biosynthetic cluster encoding spore pigment synthesis. Here we identify the first direct target for SigF, the gene sspA, encoding a sporulation-specific protein. Bioinformatic analysis suggests that SspA is a secreted lipoprotein with two PepSY signature domains. The sspA deletion mutant exhibits irregular sporulation septation and altered spore shape, suggesting that SspA plays a role in septum formation and spore maturation. The fluorescent translational fusion protein SspA-mCherry localized first to septum sites, then subsequently around the surface of the spores. Both SspA protein and sspA transcription are absent from the sigF null mutant. Moreover, in vitro transcription assay confirmed that RNA polymerase holoenzyme containing SigF is sufficient for initiation of transcription from a single sspA promoter. In addition, in vivo and in vitro experiments showed that sspA is a direct target of BldD, which functions to repress sporulation genes, including whiG, ftsZ and ssgB, during vegetative growth, co-ordinating their expression during sporulation septation.

The Ser/Thr protein kinase AfsK regulates polar growth and hyphal branching in the filamentous bacteria Streptomyces.

In cells that exhibit apical growth, mechanisms that regulate cell polarity are crucial for determination of cellular shape and for the adaptation of growth to intrinsic and extrinsic cues. Broadly conserved pathways control cell polarity in eukaryotes, but less is known about polarly growing prokaryotes. An evolutionarily ancient form of apical growth is found in the filamentous bacteria Streptomyces, and is directed by a polarisome-like complex involving the essential protein DivIVA. We report here that this bacterial polarization machinery is regulated by a eukaryotic-type Ser/Thr protein kinase, AfsK, which localizes to hyphal tips and phosphorylates DivIVA. During normal growth, AfsK regulates hyphal branching by modulating branch-site selection and some aspect of the underlying polarisome-splitting mechanism that controls branching of Streptomyces hyphae. Further, AfsK is activated by signals generated by the arrest of cell wall synthesis and directly communicates this to the polarisome by hyperphosphorylating DivIVA. Induction of high levels of DivIVA phosphorylation by using a constitutively active mutant AfsK causes disassembly of apical polarisomes, followed by establishment of multiple hyphal branches elsewhere in the cell, revealing a profound impact of this kinase on growth polarity. The function of AfsK is reminiscent of the phoshorylation of polarity proteins and polarisome components by Ser/Thr protein kinases in eukaryotes.

How c-di-GMP controls progression through the Streptomyces life cycle.

Members of the antibiotic-producing bacterial genus Streptomyces undergo a complex developmental life cycle that culminates in the production of spores. Central to control of this cell differentiation process is signaling through the second messenger 3', 5'-cyclic diguanylic acid (c-di-GMP). So far, three proteins that are directly controlled by c-di-GMP in Streptomyces have been functionally and structurally characterized: the key developmental regulators BldD and σWhiG, and the glycogen-degrading enzyme GlgX. c-di-GMP signals through BldD and σWhiG, respectively, to control the two most dramatic transitions of the Streptomyces life cycle, the formation of the reproductive aerial hyphae and their differentiation into spore chains. Later in development, c-di-GMP activates GlgX-mediated degradation of glycogen, releasing stored carbon for spore maturation.