The gene is required for the early stages of sporangium formation in .

UNLABELLED

In , 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 , which develops terminal sporangia on the substrate mycelia via short sporangiophores. Importantly, AmSsgB is the sole SALP in . The transcription of was upregulated during sporangium formation, consistent with our previous findings that is a member of the AmBldD regulon. The null mutant (Δ) 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 . Additionally, analysis of the heterologous expression of seven SALP-encoding genes from A3(2) () in the Δ strain revealed that only could compensate for AmSsgB deficiency. This indicated that SsgB of A3(2) and AmSsgB have comparable functions in . In contrast to the Δ strain, the -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 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 is essential for sporangium expansion, which does not seem to be related to spore septum formation. However, the heterologous expression of from A3(2) restored morphological abnormalities in the Δ mutant. We propose that the primary function of SsgB is to initiate sporulation in differentiating cells (e.g., aerial hyphae in and "presporangium" cells in ) although its molecular mechanism remains unknown.