Engineering a repression-free catabolite-enhanced expression system for a thermophilic alpha-amylase from Bacillus licheniformis MSG

Abstract

Most expression systems used for the over production of many enzymes employ carbon catabolite repressible promoters and hence must use sugar free, rich complex media. Use of expression systems to overcome carbon repression opens an avenue for exploiting cheap carbon sources for the production of recombinant enzymes. A self-inducible, catabolite repression free and above all a glucose-activated expression system has been developed using an industrially suitable thermophilic alpha-amylase as a model. The alpha-amylase gene of Bacillus licheniformis MSG without the 5' cre operator produced unimpeded glucose-enhanced expression when fused to the phosphate starvation-inducible strong pst promoter with optimum translation signals in a protease deficient Bacillus subtilis. A combination of high glucose with limited phosphate permitted sufficient biomass and fast transition to quiescent phase by phosphate starvation permitting 1250-fold induction for 70h. A ~300-fold high productivity (9070UmL-1) and 131-fold increase in specific expression in 2% glucose and a 100-fold high yield in 4% molasses were obtained compared to the production by the parent strain. The yield was 18.5-fold higher than that from the native promoter in an isogenic clone. This strategy of catabolite enhanced enzyme expression uncoupled from biomass formation can be applied for cost effective high production of proteins using starch or molasses.