The low molecular weight (LMW) heat shock protein (HSP) gene hsp16.6 was identified and cloned from the unicellular cyanobacterium Synechocystis sp. PCC 6803 through comparisons of genomic sequences and conserved gene sequences of the LMW HSPs. Hsp16.6 was isolated using PCR and cloned into the pGEMT plasmid. Hsp16.6 showed a significant increase in transcription after heat shock at 42 degrees C that indicated hsp16.6 was a heat shock gene. To determine the role that hsp16.6 plays in the heat shock response, a mutant Synechocystis cell line was generated. Cell growth and oxygen evolution rates of wild type and mutant cells were compared after heat shock. Results showed significantly decreased cell growth rates and a 40% reduction in oxygen evolution rates in mutants after heat shock treatments. These data indicate a protective role for hsp16.6 in the heat shock response.