Induction of heat shock proteins following transfer of E.coli cells from 30°C to 42°C depends on rapid accumulation of θ32, a minor θ factor specifically required for transcription of heat shock genes. The synthesis of θ32 is induced by enhancing translation of its mRNA transcribed from the rpoH (htpR) gene. We previously showed that the translational control of rpoH-lacZ gene fusion is mediated by two cis-acting rpoH coding regions presumably involving mRNA secondary structure. To further examine this model, we constructed and analyzed a set of gene fusions carrying base substitution(s) or internal deletions within rpoH, including constitutive mutations predicted to destroy the mRNA secondary structure and compensatory second-site mutations that may restore the secondary structure. The results demonstrate that base pairings between the translation initiation region of some 20 nucleotides and part of the internal complementary sequences are critical for maintaining repression during steady-state growth and for modulating heat-induced synthesis of θ32 -β-galactosidase fusion protein upon temperature upshift. Furthermore, some of the compensatory mutations resulted in super-repressed (non-inducible) phenotypes, suggesting that the heat induction depends on a specific nucleotide sequence(s) as well as the mRNA secondary structure within the 5'-proximal regulatory segment of rpoH coding region.