Identification and characterization of novel low-temperature-inducible promoters of Escherichia coli

Abstract

Escherichia coli promoters that are more active at low temperature (15 to 20 degrees C) than at 37 degrees C were identified by using the transposon Tn5-lac to generate promoter fusions expressing beta-galactosidase (beta-Gal). Tn5-lac insertions that resulted in low-temperature-regulated beta-Gal expression were isolated by selecting kanamycin-resistant mutants capable of growth on lactose minimal medium at 15 degrees C but which grew poorly at 37 degrees C on this medium. Seven independent mutants were selected for further studies. In one such strain, designated WQ11, a temperature shift from 37 degrees C to either 20 or 15 degrees C resulted in a 15- to 24-fold induction of beta-Gal expression. Extended growth at 20 or 15 degrees C resulted in 36- to 42-fold-higher beta-Gal expression over that of cells grown at 37 degrees C. Treatment of WQ11 with streptomycin, reported to induce a response similar to heat shock, failed to induce beta-Gal expression. In contrast, treatment with either chloramphenicol or tetracycline, which mimics a cold shock response, resulted in a fourfold induction of beta-Gal expression in strain WQ11. Hfr genetic mapping studies complemented by physical mapping indicated that in at least three mutants (WQ3, WQ6, and WQ11), Tn5-lac insertions mapped at unique sites where no known cold shock genes have been reported. The Tn5-lac insertions of these mutants mapped to 81, 12, and 34 min on the E. coli chromosome, respectively. The cold-inducible promoters from two of the mutants (WQ3 and WQ11) were cloned and sequenced, and their temperature regulation was examined. Comparison of the nucleotide sequences of these two promoters with the regulatory elements of other known cold shock genes identified the sequence CCAAT as a putative conserved motif.