ACTIVE TRANSLATION OF THE D-1 PROTEIN OF PHOTOSYSTEM-II IN SENESCING LEAVES

Primary leaves of bean (Phaseolus vulgaris L.) show pronounced symptoms of senescence within 21 days of planting. Total RNA levels decline by approximately fourfold, and pulse-labeling studies with [S-35]methionine have indicated that synthesis of all thylakoid proteins except the D-1 protein of photosystem 11 is sharply curtailed. Measurements of transcript levels for D-1, which is encoded by psbA, and for two other thylakoid proteins, the 68- to 70-kilodalton protein of photosystem I encoded by psaA/B and the beta-subunit of ATPase encoded by atpB/E, have indicated that the continued strong synthesis of D-1 relative to other proteins in senescing thylakoids does not reflect differential changes in message abundance. Specifically, although transcript quantity for each of these proteins decreases by approximately fourfold with advancing senescence, only the 68- to 70-kilodalton protein of photosystem I and the beta-subunit of ATPase show decreased synthesis. As well, the proportion of total psbA transcript associated with polysomes is higher in senescent leaves than in young leaves, whereas for each of psaA/B and atpB/E, there is less message in polysome formation in the senescent leaves than in young leaves. Thus, the continued strong synthesis of D-1 in senescent leaves appears to reflect preferential sequestering of ribosomes by psbA transcript from a dwindling ribosome pool. This ability to preferentially sequester ribosomes may be related to the unusually high turnover rate that characterizes D-1.