Role of a thermostable laccase produced by Streptomyces ipomoeae in the degradation of wheat straw lignin in solid state fermentation

Streptomycetes are actually considered one of the main groups of ligninolytic microoganisms producing a lignin-carbohydate complex named APPL (acid precipitable polymeric lignin) when growing on lignocellulosic materials. Although in these conditions the production of hemicellulolytic and oxidative extracellular enzymes were reported the specific role of laccases in lignin degradation is poorly understood. SilA, a thermostable salt-resistant and pH-versatile laccase produced by Streptomyces ipomoeae CECT 3341 was recently discovered and their particular characteristics make attractive to deep in its knowledge for biotechnological and environmental purposes. Pyrolysis/GC-MS was used to analyse the behaviour of the laccase-producing strain (SilA strain) and a laccase-negative mutant (SilA - strain) when growing on wheat straw in solid-state fermentation (SSF). Quantitative yields of APPL and the relative abundance of lignin-derived compounds were much higher for SilA strain than for SilA - showing a higher solubilizing activity of S. ipomoeae wild-type on lignocellulosic residues. Nonetheless the patterns of lignin derived compounds found in the APPL pyrograms were similar for both strains and distinct from the control showing a shortening of lignin propyl side-chains. Our results demonstrate that SilA laccase is a key enzyme in the lignin solubilization by S. ipomoeae and also points to the involvement of other oxidative enzymatic activities distinct to laccase in this process.