Stimulation of Expression of a Silica-Induced Protein (Sip) in Thermus thermophilus by Supersaturated Silicic Acid.

The effects of silicic acid on the growth of Thermds thermophilus TMY, an extreme thermophile isolated from a siliceous deposit formed from geothermal water at a geothermal power plant in Japan, were examined at 75°C. At concentrations higher than the solubility of amorphous silica (400 to 700 ppm SiO[sub2]), a silica-induced protein (Sip) was isolated from the cell envelope rraction of log-phase TMY cells grown in the presence of supersaturated silicic acid. Two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed the molecular mass and pI of Sip to be about 35 kDa and 9.5, respectively. Induction of Sip expression occurred within 1 h after the addition of a supersaturating concentration of silicic acid to TM broth. Expression of Sip-like proteins was also observed in other thermophiles, including T. thermophilus HB8 and Thermus aquaticus Yr-1. The amino acid sequence of Sip was similar to that of the predicted solute-binding protein of the Fe[sup3+] ABC transporter in T. thermophilus HB8 (locus tag, TTHA1628; GenBank accession no. NC̱006461; GenelD, 3169376). The sip gene (987-bp) product showed 87% identity with the TTHA1628 product and the presumed Fe[sup3+]-binding protein of T. thermophi1us HB27 (locus tag TTC1264; GenBank accession no. NC̱005835; GenelD, 2774619). Within the genorpe, sip is situated as a component of the Fbp-type ABC transporter operon, which contains a palindromic structure immediately downstream of sip. This structure is conserved in other T. thermophilus genomes and may function as a terminator that causes definitive Sip expression in response to silica stress. [ABSTRACT FROM AUTHOR]