2004 年 21 巻 p. 51-58
Plant cells have two different mechanisms for the assembly of iron-sulfur clusters: the mitochondrial mechanism and the chloroplastic mechanism, which is less well characterized. Cysteine desulfurase catalyzes the desulfurization of L-cysteine and is supposed to deliver the sulfur atom for the synthesis of iron-sulfur clusters in both organelles. However, it remains unclear what proteins cooperate with cysteine desulfurase for cluster assembly in chloroplasts. The protein encoded by slr0077 of Synechocystis sp. PCC6803, named SsCsd3, shows a high sequence similarity to chloroplastic cysteine desulfurase from Arabidopsis thaliana (AtCpNifS) (60% identity). Thus, the mechanism for cluster assembly involving SsCsd3 can be regarded as a model of the mechanism operating in chloroplasts. In this study, SsCsd3 was overproduced, purified, and characterized. SsCsd3 acted not only on L-cysteine but also on L-selenocysteine, although the physiological significance of its activity toward L-selenocysteine is unknown. The specific activity of purified SsCsd3 toward 10 mM L-selenocysteine (5.4 units/mg) was comparable to that of AtCpNifS (3.7 units/mg), and the activities of these enzymes toward レselenocysteine were much higher (over 100 times) than those toward L-cysteine. Thus, SsCsd3 is similar to AtCpNifS not only in its primary structure but also in its catalytic properties. The iron-sulfur cluster of ferredoxin was reconstituted in vitro by using SsCsd3 as the sulfur delivery protein.
