The htpG null mutant was obtained by inserting a chloramphenicol resistance cassette (Cm-r) in the htpG coding sequence. The htpG null mutant (DeltahtpG), Deltahsp16.6, and the double mutant, DeltahtpG::hsp16.6 cells showed little growth disadvantage at 30degreesC and 37degreesC, but not at 40degreesC. This suggests that HtpG and HSP16.6 proteins do not have an essential role during growth at normal and mildly elevated temperatures. Cell growth, cell survival rate, and oxygen electrode measurements demonstrated that DeltahtpG, Deltahsp16.6, and DeltahtpG::hsp16.6 cells were sensitive to heat stress. Decreased basal and acquired thermotolerance was observed when mutants were heat shocked, with DeltahtpG::hsp16.6 being the most sensitive. A comparison of mutants showed that Deltahsp16.6 was more sensitive to heat shock than DeltahtpG.