Yasuo Ohnishi

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 (Δ atsA ) strain formed irregular sporangia of various sizes. Transmission electron microscopy revealed that some Δ atsA sporangiospores did not mature properly. Phase-contrast microscopy revealed that sporangium dehiscence did not proceed properly in the abnormally small sporangia of the Δ atsA strain, whereas apparently normal sporangia opened to release the spores. Consistently, the number of spores released from Δ atsA sporangia was lower than that released from wild-type sporangia. These phenotypic changes were recovered by introducing atsA with its own promoter into the Δ atsA 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. IMPORTANCE Actinoplanes 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.