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

The rare actinomycete 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 ]), which is a transcriptional regulator involved in morphological development and secondary metabolism in 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 throughout growth. An 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 3',5'-Cyclic di-GMP significantly enhanced the DNA-binding ability of AmBldD. A chromatin immunoprecipitation-sequencing analysis and an 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 , , , , and orthologues. These genes are involved in morphological development in 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. The rare actinomycete 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 , have been studied outside the streptomycetes. Here, it was revealed that the BldD orthologue AmBldD is essential for normal developmental processes in The AmBldD regulon seems to be different from the BldD regulon in A3(2), but they share four genes that are involved in morphological differentiation in A3(2).