ACCUMULATION OF PLASTID HSP-23 OF CHENOPODIUM-RUBRUM IS CONTROLLED POSTTRANSLATIONALLY BY LIGHT

The expression of the 23 kDa plastid heat-shock protein (HSP) of Cheno podium rubrum has been studied at various light intensities at a tempe rature of 38 degrees C where the 23 kDa protein accumulates to its hig hest levels. It was observed that the level of mRNA which is induced a t this heat-shock temperature is independent of the light intensity be tween 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: whi le almost no protein is detectable in the dark the level of the accumu lated protein reaches a maximum at a light intensity of 300 W m(-2). T he accumulated levels of HSP 23 correlate well with resistance against photoinhibition; photoinhibitory effects are observed at a light inte nsity of 300 W m(-2) or above as measured by the decline of PS II acti vity. When high light intensities are applied during recovery from hea t 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 accumu lation of HSP 17 and 70, as analysed by Western blot, is not influence d by light, When under particular stress conditions the levels of HSP 23 remain low a protein of 31 kDa accumulates that reacts with the ant ibody to HSP 23 and might represent the precursor of HSP