Accumulation of plastid HSP 23 of Chenopodium rubrum is controlled post-translationally by light

The expression of the 23 kDa plastid heat-shock protein (HSP) of Chenopodium rubrum has been studied at various light intensities at a temperature of 38°C where the 23 kDa protein accumulates to its highest levels. It was observed that the level of mRNA which is induced at this heat-shock temperature is independent of the light intensity between 0 and 1000 W m^?2. Labelling in vivo of all investigated HSP is also not dependent on the light fluxes applied. In clear contrast the accumulation of the mature chloroplast HSP 23 is light dependent: while almost no protein is detectable in the dark the level of the accumulated protein reaches a maximum at a light intensity of 300 W m^?2. The accumulated levels of HSP 23 correlate well with resistance against photoinhibition; photoinhibitory effects are observed at a light intensity of 300 W m^?2 or above as measured by the decline of PS II activity. When high light intensities are applied during recovery from heat shock the amounts of HSP 23 stay elevated for a longer time and at a higher level than at the standard light intensity of 10 W m^?2. This appears to be a peculiar property of the plastid HSP 23 as the accumulation of HSP 17 and 70, as analysed by Western blot, is not influenced by light. When under particular stress conditions the levels of HSP 23 remain low a protein of 31 kDa accumulates that reacts with the antibody to HSP 23 and might represent the precursor of HSP 23.