Background: The use of lignocellulosic materials for second generation ethanol production would give several advantages such as minimizing the conflict between land use for food and fuel production, providing less expensive raw materials than conventional agricultural feedstock, allowing lower greenhouse gas emissions than those of first generation ethanol. However, cellulosic biofuels are not produced at a competitive level yet, mainly because of the high production costs of the cellulolytic enzymes. Therefore, this study was aimed at discovering new cellulolytic microorganisms and enzymes.
Results: Different bacteria isolated from raw composting materials obtained from vegetable processing industry wastes were screened for their cellulolytic activity on solid medium containing carboxymethylcellulose. Four strains belonging to the actinomycetes group were selected on the basis of their phenotypic traits and cellulolytic activity on solid medium containing carboxymethylcellulose. The strain showing the highest cellulolytic activity was identified by 16S rRNA sequencing as belonging to Streptomyces genus and it was designated as Streptomyces sp. strain G12. Investigating the enzymes responsible for cellulase activity produced by Streptomyces G12 by proteomic analyses, two endoglucanases were identified. Gene coding for one of these enzymes, named CelStrep, was cloned and sequenced. Molecular analysis showed that the celstrep gene has an open reading frame encoding a protein of 379 amino acid residues, including a signal peptide of 37 amino acid residues. Comparison of deduced aminoacidic sequence to the other cellulases indicated that the enzyme CelStrep can be classified as a family 12 glycoside hydrolase. Heterologous recombinant expression of CelStrep was carried out in Escherichia coli, and the active recombinant enzyme was purified from culture supernatant and characterized. It catalyzes the hydrolysis of carboxymethylcellulose following a Michaelis-Menten kinetics with a KM of 9.13 mg/ml and a vmax of 3469 μM min-1. The enzyme exhibits a half life of around 24 h and 96 h at 60°C and 50°C, respectively and shows a retention of around 80% of activity after 96 h at 40°C.
Conclusions: In this manuscript, we describe the isolation of a new cellulolytic strain, Streptomyces sp. G12, from industrial waste based compost, the identification of the enzymes putatively responsible for its cellulolytic activity, the cloning and the recombinant expression of the gene coding for the Streptomyces sp. G12 cellulase CelStrep, that was characterized showing to exhibit a relevant thermoresistance increasing its potential for cellulose conversion.