The heat shock response is generally characterized by an immediate, intense, and transient activation of gene expression, resulting in the elevated synthesis of heat shock proteins. We found that light modulates these characteristics of the heat shock response in cyanobacteria. Light accelerated the heat induction of htpG, groESL1, groEL2, and hspA, in Synechocystis sp. PCC 6803. In the dark, heat shock response of all the heat shock genes except hspA was not as intense as in the light and no transient peak was detected within 3 h after heat shock over the time course of the hspA and groESL1 mRNA accumulation. There was an apparent relationship between the enhancement of the heat shock gene transcription in the light and the level of reduced plastoquinone in the photosynthetic electron transport system. Light affected the transcription, but not the stability of the mRNA of heat shock genes, although the stability was quite different, depending on the heat shock gene. Light also enhanced both the accumulation of GroEL under heat stress and the acquired thermo-tolerance. (C) 2003 Elsevier Science (USA). All rights reserved.