Mutations in the regulatory regions result in increased streptomycin resistance and keratinase synthesis in Bacillus thuringiensis

Abstract

Keratinases are a group of proteases of great industrial significance. To take full advantage of Bacillus species as an inherent superior microbial producer of proteases, we performed the ribosome engineering to improve the keratinase synthesis capacity of the wild-type Bacillus thuringiensis by inducing streptomycin resistance. Mutant Bt(Str-O) was identified as a stable keratinase overproducer. Comparative characterization of the two strains revealed that, although the resistance to Streptomycin increased by eight-fold in MIC, the mutant’s resistance to other commonly used antibiotics was not affected. Furthermore, the mutant exhibited an enhanced keratinase synthesis (1.5-fold) when cultured in a liquid LB medium. In the whole feather degradation experiment, the mutant could secret twofold keratinase into the medium, reaching 640 U/mL per 10⁷ CFU. By contrast, no significant differences were found in the scanning electron microscopic analysis and spore formation experiment. To understand the genetic factors causing these phenotypic changes, we cloned and analyzed the rpsL gene. No mutation was observed. We subsequently determined the genome sequences of the two strains. Comparing the rpsL gene revealed that the emergence of streptomycin resistance was not necessarily dependent on the mutation(s) in the generally recognized “hotspot.” Genome-wide analysis showed that the phenotypic changes of the mutant were the collective consequence of the genetic variations occurring in the regulatory regions and the non-coding RNA genes. This study demonstrated the importance of genetic changes in regulatory regions and the effectiveness of irrational ribosome engineering in creating prokaryotic microbial mutants without sufficient genetic information.