Involvement of three FliA-family sigma factors in the sporangium formation, spore dormancy and sporangium dehiscence in Actinoplanes missouriensis

Involvement of a putative acyltransferase gene in sporangium formation in Actinoplanes missouriensis

The actinomycete Actinoplanes missouriensis forms branched substrate mycelia during vegetative growth and produces terminal sporangia, each of which contains a few hundred spherical flagellated spores, from the substrate mycelia through short sporangiophores. Based on the observation that remodeling of membrane lipid composition is involved in the morphological development of Streptomyces coelicolor A3(2), we hypothesized that remodeling of membrane lipid composition is also involved in sporangium formation in A. missouriensis. Because some acyltransferases are presumably involved in the remodeling of membrane lipid composition, we disrupted each of the 22 genes annotated as encoding putative acyltransferases in the A. missouriensis genome and evaluated their effects on sporangium formation. The atsA (AMIS_52390) null mutant (ΔatsA) strain formed irregular sporangia of various sizes. Transmission electron microscopy revealed that some ΔatsA sporangiospores did not mature properly. Phase-contrast microscopy revealed that sporangium dehiscence did not proceed properly in the abnormally small sporangia of the ΔatsA strain, whereas apparently normal sporangia opened to release the spores. Consistently, the number of spores released from ΔatsA sporangia was lower than that released from wild-type sporangia. These phenotypic changes were recovered by introducing atsA with its own promoter into the ΔatsA strain. These results demonstrate that AtsA is required for normal sporangium formation in A. missouriensis, although the involvement of AtsA in the remodeling of membrane lipid composition is unlikely because AtsA is an acyltransferase_3 (AT3) protein, which is an integral membrane protein that usually catalyzes the acetylation of cell surface structures.IMPORTANCEActinoplanes missouriensis goes through a life cycle involving complex morphological development, including mycelial growth, sporangium formation and dehiscence, swimming as zoospores, and germination to mycelial growth. In this study, we carried out a comprehensive gene disruption experiment of putative acyltransferase genes to search for acyltransferases involved in the morphological differentiation of A. missouriensis. We revealed that a stand-alone acyltransferase_3 domain-containing protein, named AtsA, is required for normal sporangium formation. Although the molecular mechanism of AtsA in sporangium formation, as well as the enzymatic activity of AtsA, remains to be elucidated, the identification of a putative acyltransferase involved in sporangium formation is significant in the study of morphological development of A. missouriensis. This finding will contribute to our understanding of a complex system for producing sporangia, a rare multicellular organism in bacteria.

The ssgB gene is required for the early stages of sporangium formation in Actinoplanes missouriensis

In Streptomyces, multiple paralogs of SsgA-like proteins (SALPs) are involved in spore formation from aerial hyphae. However, the functions of SALPs have not yet been elucidated in other actinobacterial genera. Here, we report the primary function of an SsgB ortholog (AmSsgB) in Actinoplanes missouriensis, which develops terminal sporangia on the substrate mycelia via short sporangiophores. Importantly, AmSsgB is the sole SALP in A. missouriensis. The transcription of AmssgB was upregulated during sporangium formation, consistent with our previous findings that AmssgB is a member of the AmBldD regulon. The AmssgB null mutant (ΔAmssgB) strain formed non-globose irregular structures on the substrate mycelium. Transmission electron microscopy revealed that the irregular structures contained abnormally septate hypha-like cells, without an intrasporangial matrix. These phenotypic changes were restored by complementation with AmssgB. Additionally, analysis of the heterologous expression of seven SALP-encoding genes from Streptomyces coelicolor A3(2) (ssgA-G) in the ΔAmssgB strain revealed that only ssgB could compensate for AmSsgB deficiency. This indicated that SsgB of S. coelicolor A3(2) and AmSsgB have comparable functions in A. missouriensis. In contrast to the ΔAmssgB strain, the ftsZ-disrupted strain showed a severe growth defect and produced small sporangium-like structures that swelled to some extent. These findings indicate that AmSsgB is crucial for the early stages of sporangium formation, not for spore septum formation in the late stages. We propose that AmSsgB is involved in sporangium formation by promoting the expansion of the "presporangium" structures formed on the tips of the substrate hyphae.

IMPORTANCE

SsgB has been proposed as an archetypical SsgA-like protein with an evolutionarily conserved function in the morphological development of spore-forming actinomycetes. SsgB in Streptomyces coelicolor A3(2) is involved in spore septum formation. However, it is unclear whether this is the primary function of SsgBs in actinobacteria. This study demonstrated that the SsgB ortholog (AmSsgB) in Actinoplanes missouriensis is essential for sporangium expansion, which does not seem to be related to spore septum formation. However, the heterologous expression of ssgB from S. coelicolor A3(2) restored morphological abnormalities in the ΔAmssgB mutant. We propose that the primary function of SsgB is to initiate sporulation in differentiating cells (e.g., aerial hyphae in Streptomyces and "presporangium" cells in A. missouriensis) although its molecular mechanism remains unknown.

A unique sigma/anti-sigma system in the actinomycete Actinoplanes missouriensis

Bacteria of the genus Actinoplanes form sporangia that contain dormant sporangiospores which, upon contact with water, release motile spores (zoospores) through a process called sporangium dehiscence. Here, we set out to study the molecular mechanisms behind sporangium dehiscence in Actinoplanes missouriensis and discover a sigma/anti-sigma system with unique features. Protein σSsdA contains a functional sigma factor domain and an anti-sigma factor antagonist domain, while protein SipA contains an anti-sigma factor domain and an anti-sigma factor antagonist domain. Remarkably, the two proteins interact with each other via the anti-sigma factor antagonist domain of σSsdA and the anti-sigma factor domain of SipA. Although it remains unclear whether the SipA/σSsdA system plays direct roles in sporangium dehiscence, the system seems to modulate oxidative stress responses in zoospores. In addition, we identify a two-component regulatory system (RsdK-RsdR) that represses initiation of sporangium dehiscence.

Involvement of BldC in the Formation of Physiologically Mature Sporangium in Actinoplanes missouriensis

AmBldD is a global transcriptional regulator that represses the transcription of several genes required for sporangium formation in Actinoplanes missouriensis. Here, we characterized one of the AmBldD regulons: AMIS_1980, encoding an ortholog of BldC, which is a transcriptional regulator involved in the morphological development of Streptomyces. We determined the transcriptional start point of the bldC ortholog by high-resolution S1 nuclease mapping and found an AmBldD box in its 5'-untranslated region. Reverse transcription-quantitative PCR analysis revealed that the transcription of bldC is activated during sporangium formation. A bldC null mutant (ΔbldC) strain formed normally shaped sporangia, but they exhibited defective sporangium dehiscence; under a dehiscence-inducing condition, the number of spores released from the sporangia of the ΔbldC strain was 2 orders of magnitude lower than that from the sporangia of the wild-type strain. RNA sequencing analysis indicated that BldC functions as a transcriptional activator of several developmental genes, including tcrA, which encodes a key transcriptional activator that regulates sporangium formation, sporangium dehiscence, and spore dormancy. Using electrophoretic mobility shift assay (EMSA), we showed that a recombinant BldC protein directly binds to upstream regions of at least 18 genes, the transcription of which is downregulated in the ΔbldC strain. Furthermore, using DNase I footprinting and EMSA, we demonstrated that BldC binds to the direct repeat sequences containing an AT-rich motif. Thus, BldC is a global regulator that activates the transcription of several genes, some of which are likely to be required for sporangium dehiscence. IMPORTANCE BldC is a global transcriptional regulator that acts as a "brake" in the morphological differentiation of Streptomyces. BldC-like proteins are widely distributed throughout eubacteria, but their orthologs have not been studied outside streptomycetes. Here, we revealed that the BldC ortholog in Actinoplanes missouriensis is essential for sporangium dehiscence and that its regulon is different from the BldC regulon in Streptomyces venezuelae, suggesting that BldC has evolved to play different roles in morphological differentiation between the two genera of filamentous actinomycetes.

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

Diverse bacterial lifestyle transitions are controlled by the nucleotide second messenger c-di-GMP, including virulence, motility, and biofilm formation. To control such fundamentally distinct processes, the set of genes under c-di-GMP control must have gone through several shifts during bacterial evolution. Here we show that the same σ–(c-di-GMP)–anti-σ switch has been co-opted during evolution to regulate distinct biological functions in unicellular and filamentous bacteria, controlling type IV pilus production in the genus Rubrobacter and the differentiation of reproductive hyphae into spores in Streptomyces. Moreover, we show that the anti-σ likely originated as a homodimer and evolved to become a monomer through an intragenic duplication event. This study thus describes the structural and functional evolution of a c-di-GMP regulatory 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.

Regulation of Sporangium Formation, Spore Dormancy, and Sporangium Dehiscence by a Hybrid Sensor Histidine Kinase in Actinoplanes missouriensis: Relationship with the Global Transcriptional Regulator TcrA

The rare actinomycete Actinoplanes missouriensis forms terminal sporangia containing a few hundred flagellated spores. In response to water, the sporangia open and release the spores into external environments. The orphan response regulator TcrA functions as a global transcriptional activator during sporangium formation and dehiscence. Here, we report the characterization of an orphan hybrid histidine kinase, HhkA. Sporangia of an hhkA deletion mutant contained many distorted or ectopically germinated spores and scarcely opened to release the spores under sporangium dehiscence-inducing conditions. These phenotypic changes are quite similar to those observed in a tcrA deletion mutant. Comparative RNA sequencing analysis showed that genes controlled by HhkA mostly overlap TcrA-regulated genes. The direct interaction between HhkA and TcrA was suggested by a bacterial two-hybrid assay, but this was not conclusive. The phosphorylation of TcrA using acetyl phosphate as a phosphate donor markedly enhanced its affinity for the TcrA box sequences in the electrophoretic mobility shift assay. Taking these observations together with other results, we proposed that HhkA and TcrA compose a cognate two-component regulatory system, which controls the transcription of the genes involved in many aspects of morphological development, including sporangium formation, spore dormancy, and sporangium dehiscence in A. missouriensisIMPORTANCEActinoplanes missouriensis goes through complex morphological differentiation, including formation of flagellated spore-containing sporangia, sporangium dehiscence, swimming of zoospores, and germination of zoospores to filamentous growth. Although the orphan response regulator TcrA globally activates many genes required for sporangium formation, spore dormancy, and sporangium dehiscence, its partner histidine kinase remained unknown. Here, we analyzed the function of an orphan hybrid histidine kinase, HhkA, and proposed that HhkA constitutes a cognate two-component regulatory system with TcrA. That HhkA and TcrA homologues are highly conserved among the genus Actinoplanes and several closely related rare actinomycetes indicates that this possible two-component regulatory system is employed for complex morphological development in sporangium- and/or zoospore-forming rare actinomycetes.