Secretory expression of a multifunctional nuclease Nuc A in <i>Bacillus licheniformis</i> 2709

Serratia nuclease Nuc A is a non-specific nucleotide hydrolase that has been widely used in large-scale protein purification or eliminating nucleic acid contamination from purified proteins. To enhance the enzyme production, the Serratia nuclease gene was synthesized and expressed in Bacillus licheniformis 2709, a robust strain capable of secreting native and heterologous proteins selectively or non-selectively. To further increase the secretory expression level of the enzyme, different strong promoters and signal peptides were fused with the mature Nuc A-encoding gene at various genetic loci. The highest expression level of Nuc A was observed under the control of regulatory elements P_aprE, which occur naturally in B. licheniformis 2709 for the alkaline protease (AprE) expression. Through maximizing the number of copies of P_aprE-nucA expression cassette at different integration sites, the yield of nuclease Nuc A reached approximately 31954 U/mL after 60 hours of cultivation in shake flasks. The specific activity of the recombinant nuclease reached 1.58×107 U/mg, which is about 9 times higher than that expressed in Escherichia coli strain. Additionally, the recombinant Nuc A exhibited high catalytic activities in the pH range of 7-10. Furthermore, it was resistant to 0.2% SDS, 1.0 mM PMSF, and 0.4% Triton X-100. After 8 M Urea treatment, residual activity is measured. The high expression levels and positive characteristics of recombinant Nuc A provide an effective solution for large-scale production and industrial application of the nuclease.