UV mutagenesis treatment for improved production of endoglucanase and β-glucosidase from newly isolated thermotolerant actinomycetes, Streptomyces griseoaurantiacus

Abstract

Bioconversion of cellulosic biomass into fuel ethanol involves several steps, among which enzymatic breakdown of cellulose into fermentable sugars play a significant role. The key enzymes involved in cellulosic breakdown are mainly endoglucanases and β-glucosidases. Even though the biochemical and molecular characterization of number of endoglucanases and β-glucosidases was extensively studied, still there is a demand for novel microbial cellulases for industrial applications. Among the group of actinomycetes, Streptomyces spp. are well known as a cellulase producer. The advantage of using actinomycetes is being that production process could be easily scaled-up to commercial levels. However, recent research studies have shown that the production of cellulases from actinomycetes could also be significantly improved by employing different types of strain improvement methods, thus achieving high yields of extracellular proteins. Besides this, highly thermostable and broad pH range cellulases are required for bioethanol application. A lignocellulose degrading actinomycetes strain was newly isolated and identified as Streptomyces griseoaurantiacus. Strain improvement using UV mutagenesis developed two mutants (SGUV30 and SGUV5) with 57.4 % and 12.8 % higher endoglucanase and β-glucosidase activities. The cellulases (endoglucanases and β-glucosidases) were found to be highly thermostable with no loss in enzyme activities at 80 °C for 60 min and nearly 80 % of initial activity was retained at 90 °C. Enzyme assays in presence of additives showed that CoCl2, CaCl2, and FeSO4 increased β-glucosidase activity but showed negative effect on endoglucanase activity. However, both the enzyme activities were significantly enhanced by addition of PEG 8000, sodium azide and MnSO4. Strain improvement of S. griseoaurantiacus was performed by UV mutagenesis where two mutant strains (SGUV30 and SGUV5) were developed with improved endoglucanase and β-glucosidase activities. Cellulase production in submerged fermentation was carried out using a cheap lignocellulosic biomass residue, rice straw as a sole source carbon. The results clearly show that the mutant strains produced high-efficient cellulases that are stable at a broad pH range at very high temperatures. Besides, the mutants also showed high extracellular protein secretions, which could be promising in reducing the overall cellulase production costs at large scale.