Molecular basis for decline of leaf function during senescence: Analysis of major determinants causing decline of the amount of Rubisco

1) Rubisco在扩张过程中合成活跃，但在完全扩张前Rubisco的合成量迅速下降。在秋季扩张时已经下降到最大值的五分之一左右，在衰老时进一步下降到十分之一左右。rbcL和rbcS mrna水平的变化几乎与Rubisco合成量的变化一致。因此，Rubisco的合成量主要取决于叶片整个生命周期中rbcL和rbcS mrna的水平。在衰老过程中，Rubisco的降解远比Rubisco的合成活跃。由于衰老过程中Rubisco的合成对其数量几乎没有贡献，因此Rubisco的降解是衰老叶片中Rubisco数量的主要决定因素。2) Rubisco （LSU）的大亚基在gy -329处被叶绿体或完整叶绿体照射裂解物中产生的活性氧直接裂解为37-kDa和16-kDa片段。通过增加孵育时间或Fe^<2+>浓度，LSU也可以被切割成若干其他片段。所有的解理位点都位于或非常靠近活性位点，就在以12Å为半径的金属结合位点周围，这表明邻近性和有利的取向可能是决定解理位点的最重要参数。GS2也被活性氧降解，降解方式与LSU相同。

1) Rubisco was actively synthesized during expansion, but the amount of Rubisco synthesized rapidly declined just before full expansion. It already declined to about one fifth of the maximal amount at the time of fall expansion, and further declined to about one-tenth during senescence. The changes in the levels of rbcL and rbcS mRNAs were almost coordinated with those changes in the amount of Rubisco synthesized. Thus, the amount of Rubisco synthesized was primarily determined by the levels of rbcL and rbcS mRNAs throughout the life span of the leaves. Degradation of Rubisco became far more active than synthesis of Rubisco during senescence. Since the synthesis of Rubisco during senescence scarcely contributed to its amount, the degradation of Rubisco is the major determinant for the amount of Rubisco in the senescent leaves.2) The large subunit of Rubisco (LSU) was directly fragmented at Gly-329 into the 37-kDa and 16-kDa fragments by reactive oxygen species generated in the illuminated lysates of chloroplasts or intact chloroplasts. The LSU could also be cleaved into several other fragments by increasing the incubation time or the Fe^<2+> concentrations. All the cleavage sites were at or very close to the active site, just around the metal binding site within a radius of 12Å, indicating that proximity and favorable orientation are probably the most important parameter determining the cleavage sites. GS2 was also degraded by reactive oxygen species with the same manner as the LSU.