TEMPERATURE-SENSITIVITY OF D1 PROTEIN-METABOLISM IN ISOLATED ZEA-MAYS CHLOROPLASTS

The effects of low temperature on the synthesis and stability of the 32 kDa D1 protein of photosystem II were investigated in chloroplasts isolated from maize (Zea mays cv. LG11) leaves. The synthesis of D1 by intact chloroplasts in vitro was strongly dependent on temperature; the Q(10) for the initial rate of incorporation of [S-35]-methionine into D1 was ca. 2.6 over the range 13-25 degrees C. The synthesis of other thylakoid polypeptides exhibited a similar temperature dependence, whilst synthesis of stromal proteins was considerably less temperature-dependent, with the exception of two polypeptides of ca. 56 and 59.5 kDa. The stability of newly-synthesized D1 in the thylakoid membranes was dependent both on the temperature at which the plants were grown and on the temperature during the pulse-labelling period when the protein was synthesized. In chloroplasts isolated from maize leaves grown at 25 OC, D1 that was synthesized and assembled at 25 degrees C in vitro was rapidly degraded during the chase period. At lower chase temperatures the protein was more stable. When chloroplasts from 25 degrees C-grown leaves were pulse-labelled at 13 degrees C, the stability of D1 was markedly enhanced at all temperatures during the chase period. This effect was even more pronounced in chloroplasts isolated from plants grown at 14 degrees C. The implications of these results are discussed with regard to the ability of maize to recover from photoinhibitory damage at low temperatures.