Fig. 1

a Construction of the expression vector pSix1 for Thermus strains. The plasmid pYK596, which possesses a hygromycin resistance gene, was digested using EcoRI and Nhe I. Two PCR-amplified fragments, a multi-cloning site derived from pET21a, and a putative silica-inducible protein (sip) promoter region [12] were cloned into pYK596 using an In-Fusion cloning kit. The Xho I site on the pYK596 backbone was deleted by inverse PCR, and the resultant plasmid was named pSix1. To complete the Miller assay, a thermostable β-galactosidase gene from the Thermus sp. A4 was cloned downstream of the sip promoter of pSix1 to yield βgal/pSix1. b Sequence of pSix1. The − 35 and − 10 regions of the sip promoter are underlined. The experimentally determined transcription initiation site (+1) is indicated in bold, and the putative ribosome binding site (rbs) is underlined. Restriction sites in the multi-cloning site and 6× histidine tag sequences are indicated above the sequence