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.

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.

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

The rare actinomycete Actinoplanes missouriensis forms sporangia, which open up and release zoospores in response to water. Here, we report a genetic and functional analysis of four FliA-family sigma factors, FliA1, FliA2, FliA3 and FliA4. Transcription of fliA1, fliA2 and fliA3 was directly activated by the global transcriptional activator TcrA during sporangium formation and dehiscence, while fliA4 was almost always transcribed at low levels. Gene disruption analysis showed that (a) deletion of fliA2 reduced the zoospore swimming speed by half, (b) the fliA1-fliA2 double-deletion mutant formed abnormal sporangia in which mutant spores ectopically germinated and (c) deletion of fliA3 induced no phenotypic changes in the wild-type and mutant strains of fliA1 and/or fliA2. Comparative RNA-Seq analyses among the wild-type and gene deletion mutant strains showed probable targets of each FliA-family sigma factor, indicating that FliA1- and FliA2-dependent promoters are quite similar to each other, while the FliA3-dependent promoter is somewhat different. Gene complementation experiments also indicated that the FliA1 regulon overlaps with the FliA2 regulon. These results demonstrate that A. missouriensis has developed a complex transcriptional regulatory network involving multiple FliA-family sigma factors for the accomplishment of its characteristic reproduction process, including sporangium formation, spore dormancy and sporangium dehiscence.

Regulation of Sporangium Formation by BldD in the Rare Actinomycete Actinoplanes missouriensis

The rare actinomycete Actinoplanes missouriensis forms sporangia, including hundreds of flagellated spores that start swimming as zoospores after their release. Under conditions suitable for vegetative growth, zoospores stop swimming and germinate. A comparative proteome analysis between zoospores and germinating cells identified 15 proteins that were produced in larger amounts in germinating cells. They include an orthologue of BldD (herein named AmBldD [BldD of A. missouriensis]), which is a transcriptional regulator involved in morphological development and secondary metabolism in Streptomyces AmBldD was detected in mycelia during vegetative growth but was barely detected in mycelia during the sporangium-forming phase, in spite of the constant transcription of AmbldD throughout growth. An AmbldD mutant started to form sporangia much earlier than the wild-type strain, and the resulting sporangia were morphologically abnormal. Recombinant AmBldD bound a palindromic sequence, the AmBldD box, located upstream from AmbldD 3',5'-Cyclic di-GMP significantly enhanced the in vitro DNA-binding ability of AmBldD. A chromatin immunoprecipitation-sequencing analysis and an in silico search for AmBldD boxes revealed that AmBldD bound 346 genomic loci that contained the 19-bp inverted repeat 5'-NN(G/A)TNACN(C/G)N(G/C)NGTNA(C/T)NN-3' as the consensus AmBldD-binding sequence. The transcriptional analysis of 27 selected AmBldD target gene candidates indicated that AmBldD should repress 12 of the 27 genes, including bldM, ssgB, whiD, ddbA, and wblA orthologues. These genes are involved in morphological development in Streptomyces coelicolor A3(2). Thus, AmBldD is a global transcriptional regulator that seems to repress the transcription of tens of genes during vegetative growth, some of which are likely to be required for sporangium formation.IMPORTANCE The rare actinomycete Actinoplanes missouriensis undergoes complex morphological differentiation, including sporangium formation. However, almost no molecular biological studies have been conducted on this bacterium. BldD is a key global regulator involved in the morphological development of streptomycetes. BldD orthologues are highly conserved among sporulating actinomycetes, but no BldD orthologues, except one in Saccharopolyspora erythraea, have been studied outside the streptomycetes. Here, it was revealed that the BldD orthologue AmBldD is essential for normal developmental processes in A. missouriensis The AmBldD regulon seems to be different from the BldD regulon in Streptomyces coelicolor A3(2), but they share four genes that are involved in morphological differentiation in S. coelicolor A3(2).