Mannose- and Mannobiose-Specific Responses of the Insect-Associated Cellulolytic Bacterium Streptomyces sp. Strain SirexAA-E

• Published in: Applied and environmental microbiology 2021-06, Vol.87 (14), p.e0271920-e0271920
• Main Authors: Ohashi, Keisuke, Hataya, Shogo, Nakata, Akane, Matsumoto, Kazuki, Kato, Natsumi, Sato, Wakana, Carlos-Shanley, Camila, Beebe, Emily T, Currie, Cameron R, Fox, Brian G, Takasuka, Taichi E
• Format: Article
• Language: English
• Subjects: Animals; Bacterial Proteins - genetics; Bacterial Proteins - metabolism; Carboxymethylcellulose Sodium - metabolism; Galactans - metabolism; Galactose - analogs & derivatives; Genetics and Molecular Biology; Insecta - microbiology; Mannans - metabolism; Mannose - metabolism; Mannosidases - genetics; Mannosidases - metabolism; Plant Gums - metabolism; Streptomyces - growth & development; Streptomyces - metabolism; Transcription Factors - genetics; Transcription Factors - metabolism
• ISSN: 0099-2240; 1098-5336
• DOI: 10.1128/AEM.02719-20

The cellulolytic insect symbiont bacterium sp. strain SirexAA-E secretes a suite of arbohydrate- ctive en ymes (CAZymes), which are involved in the degradation of various polysaccharides in the plant cell wall, in response to the available carbon sources. Here, we examined a poorly understood response of this bacterium to mannan, one of the major plant cell wall components. SirexAA-E grew well on mannose, carboxymethyl cellulose (CMC), and locust bean gum (LBG) as sole carbon sources in the culture medium. The secreted proteins from each culture supernatant were tested for their polysaccharide-degrading ability, and the composition of secreted CAZymes in each sample was determined by liquid chromatography-tandem mass spectrometry (LC-MS/MS). The results indicated that mannose, LBG, and CMC induced the secretion of mannan and cellulose-degrading enzymes. Interestingly, two α-1,2-mannosidases were abundantly secreted during growth on mannose and LBG. Using genomic analysis, we found a unique 12-bp palindromic sequence motif at 4 locations in the SirexAA-E genome, two of which were found upstream of the above-mentioned α-1,2-mannosidase genes, along with a newly identified mannose and mannobiose-responsive transcriptional regulator, SsManR. Furthermore, the previously reported cellobiose-responsive repressor, SsCebR, was determined to also use mannobiose as an effector ligand. To test whether mannobiose induces the sets of genes under the control of the two regulators, SirexAA-E was grown on mannobiose, and the secretome composition was analyzed. As hypothesized, the composition of the mannobiose secretome combined sets of CAZymes found in both LBG and CMC secretomes, and thus they are likely under the regulation of both SsManR and SsCebR. sp. SirexAA-E, a microbial symbiont of biomass-harvesting insects, secretes a suite of polysaccharide-degrading enzymes dependent on the available carbon sources. However, the response of this bacterium to mannan has not been documented. In this study, we investigated the response of this bacterium to mannose, mannobiose, and galactomannan (LBG). By combining biochemical, proteomic, and genomic approaches, we discovered a novel mannose and mannobiose responsive transcriptional regulator, SsManR, which selectively regulates three α-1,2-mannosidase-coding genes. We also demonstrated that the previously described cellobiose responsive regulator, SsCebR, could use mannobiose as an effector ligand. Overall, our findings suggest