Production improvement and photoprotection of melanin produced by Bacillus thuringiensis

In our previous research, we constructed a Bacillus thuringiensis (Berliner) (Bacillales: Bacillaceae; Bt) strain (BMB171ΔhmgA), in which the loss of homogentisate 1, 2-dioxygenase (HmgA) activity promoted the melanin accumulation in the culture medium. However, further improvement in melanin yield by Bt strains has not been considered. Herein, based on the melanin synthesis pathway, we attempted to increase the melanin yield in Bt by adding a suitable concentration of L-tyrosine and increasing the expression levels of tyrosine aminotransferase (PhhC) and 4-hydroxyphenylpyruvate dioxygenase (HppD) using strong promoters. The results showed that the melanin yield of the engineered strain (Bt-2P) grown in medium supplemented with L-tyrosine was approximately 5.7 times higher than that of the original strain without L-tyrosine, suggesting that L-tyrosine addition and phhC or hppD overexpression were effective to improve the melanin yield in Bt. In addition, we tested the photoprotective effect of melanin on Autographa californica (Speyer) (Lepidoptera: Noctuidae) multiple nucleopolyhedrovirus (Lefavirales: Baculoviridae; AcMNPV) and Ralstonia solanacearum (Smith) Yabuuchi et al. (Burkholderiales: Burkholderiaceae) bacteriophage ФNK-7 against ultraviolet (UV) irradiation. The results implied that the titers of AcMNPV and bacteriophage ФNK-7 in the presence of melanin were approximately ten and four times higher than those without melanin under UV radiation, respectively. This indicated that the addition of melanin could effectively maintain the activities of the viruses. In conclusion, we reported strategies to increase the melanin yield in Bt and expanded the potential applications of melanin.