Development of a strictly regulated xylose-induced expression system in Streptomyces.

Background: Genetic tools including constitutive and inducible promoters have been developed over the last few decades for strain engineering in Streptomyces. Inducible promoters are useful for controlling gene expression, however only a limited number are applicable to Streptomyces. The aim of this study is to develop a controllable protein expression system based on an inducible promoter using sugar inducer, which has not yet been widely applied in Streptomyces.

Results: To determine a candidate promoter, inducible protein expression was first examined in Streptomyces avermitilis MA-4680 using various carbon sources. Xylose isomerase (xylA) promoter derived from xylose (xyl) operon was selected due to strong expression of xylose isomerase (XylA) in the presence of D-xylose. Next, a xylose-inducible protein expression system was constructed by investigating heterologous protein expression (chitobiase as a model protein) driven by the xylA promoter in Streptomyces lividans. Chitobiase activity was detected at high levels in S. lividans strain harboring an expression vector with xylA promoter (pXC), under both xylose-induced and non-induced conditions. Thus, S. avermitilis xylR gene, which encodes a putative repressor of xyl operon, was introduced into constructed vectors in order to control protein expression by D-xylose. Among strains constructed in the study, XCPR strain harboring pXCPR vector exhibited strict regulation of protein expression. Chitobiase activity in the XCPR strain was observed to be 24 times higher under xylose-induced conditions than that under non-induced conditions.

Conclusion: In this study, a strictly regulated protein expression system was developed based on a xylose-induced system. As far as we could ascertain, this is the first report of engineered inducible protein expression in Streptomyces by means of a xylose-induced system. This system might be applicable for controllable expression of toxic products or in the field of synthetic biology using Streptomyces strains.