Sigma Factor Engineering in Sp. SE50/110: Expression of the Alternative Sigma Factor Gene (σH) Enhances Acarbose Yield and Alters Cell Morphology

Sigma factors are transcriptional regulators that are part of complex regulatory networks for major cellular processes, as well as for growth phase-dependent regulation and stress response. sp. SE50/110 is the natural producer of acarbose, an α-glucosidase inhibitor that is used in diabetes type 2 treatment. Acarbose biosynthesis is dependent on growth, making sigma factor engineering a promising tool for metabolic engineering. ACSP50_0507 is a homolog of the developmental and osmotic-stress-regulating σH. Therefore, the protein encoded by was named σH. Here, an sp. SE50/110 expression strain for the alternative sigma factor gene () achieved a two-fold increased acarbose yield with acarbose production extending into the stationary growth phase. Transcriptome sequencing revealed upregulation of acarbose biosynthesis genes during growth and at the late stationary growth phase. Genes that are transcriptionally activated by σH frequently code for secreted or membrane-associated proteins. This is also mirrored by the severely affected cell morphology, with hyperbranching, deformed and compartmentalized hyphae. The dehydrated cell morphology and upregulation of further genes point to a putative involvement in osmotic stress response, similar to its homolog. The DNA-binding motif of σH was determined based on transcriptome sequencing data and shows high motif similarity to that of its homolog. The motif was confirmed by in vitro binding of recombinantly expressed σH to the upstream sequence of a strongly upregulated gene. Autoregulation of σH was observed, and binding to its own gene promoter region was also confirmed.