Actinobacterial Strains as Genomic Candidates for Characterization of Genes Encoding Enzymes in Bioconversion of Lignocellulose

Many soil Actinobacteria are potent producers of extracellular enzymes decomposing lignocellulose. Four strains of Actinobacteria with a high potential to hydrolyse cellulose and hemicellulose were identified among environmental isolates. The strains were grown on raw lignocellulosic substrates (olive pomace, oat flakes, sawdust, and wheat straw) under submerged fermentation in a laboratory scale. Modified Melin Norkrans Medium amended with raw lignocellulosic substrates as carbon sources (0.5%) was used to enhance lignocellulosic biomass decomposition. Three strains belonged to the genus Streptomyces and one strain to the genus Mycobacterium . Annotation of genomes showed high proportion of genes encoding for carbohydrate-active enzymes in Streptomyces sp. GESEQ-4 (537, i.e. 6% of 8404 genes), Streptomyces sp. GESEQ-13 (351 (5%) of 6705 genes), Streptomyces sp. GESEQ-35 (608 (6%) of 9788 genes), and Mycolicibacterium fortuitum subsp. fortuitum GESEQ-9 (222 (3%) of 6405 genes). These included plant cell wall-degrading enzymes belonging to the families GH1, GH2, GH3, GH5, GH6, GH9, GH10, GH12, GH16, GH26, GH30, GH39, GH48, GH51, and GH74, of which GH1, GH2, GH3, GH5, GH6, and GH16 were found in all four genomes. Assays for cellulose and hemicellulose degrading extracellular enzymes confirmed the ability of the isolates to decompose cellulose and hemicellulose. The highest endo-cleaving enzyme activities were produced by the strain Steptomyces sp. GESEQ-4 DSM 106287. Our results provide new perspectives into the enzymatic array by which the Actinobacteria break down complex lignocellulosic biomass. It is crucial to assess the genome to determine enzyme function as well as the enzyme families responsible for the degradation process in Actinobacteria. The potential degradation functions for these actinobacterial strains were validated by testing their cellulolytic and hemicellulolytic activities with various lignocellulosic substrates. Graphic Abstract