Northern blot analysis of hrcA, grpE, dnaK, and dnaJ mRNAs in response to heat shock. RNA was isolated from Listeria monocytogenes 10403S grown at 30°C (0 minutes) and at 10 or 20 minutes after a temperature shift to 45°C. The filters were hybridized with labeled DNAs corresponding to the hrcA (A), grpE (B), dnaK (C), and dnaJ (D). The size of each transcript was determined by comparison with RNA size markers