A novel 51-kDa fragment of the large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase formed in the stroma of chloroplasts in dark-induced senescing wheat leaves.

The degradation of large subunit (LSU) of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) in wheat (Triticum aestivum L. cv. Yangmai 158) leaves was studied. A novel 51-kDa fragment was detected in leaf crude extracts and in chloroplast lysates from leaves with dark-induced senescence. Further studies showed that the 51-kDa fragment was found in the reaction solution with stroma fraction but not in that with the chloroplast membrane fraction and in the chloroplast lysates from mature wheat leaves. The reaction of producing the 51-kDa fragment was inhibited by 4-(2-aminoethyl) benzenesulfonyl fluoride hydrochloride (AEBSF), 1,10-phenanthroline and EDTA. The N-terminal sequence analysis indicated that the LSU was cleaved at the peptide bond between Lys-14 and Ala-15. In addition, a 50-kDa fragment of LSU formed obviously at pH 6.0-6.5 was detected in the crude extracts of leaves with dark-induced senescence but was not found in lysates of chloroplasts. The degradation was prevented by AEBSF, leupeptin and transepoxysuccinyl-l-leucylamido (4-guanidino) butane (E-64). The results obtained in this study imply that the appearance of the 51-kDa fragment could be because of the involvement of a new senescence-associated protease that is located in the stroma of chloroplasts in senescing wheat leaves.