Arabidopsis SUFB and CLP protease regulate SUFBC2D to manipulate iron-sulfur cluster biosynthesis in chloroplast

Abstract Iron‒sulfur (Fe‒S) clusters are essential cofactors for Fe‒S proteins. SUFBC 2 D complex is the scaffold responsible for Fe‒S cluster assembly in chloroplasts. However, the regulatory mechanism on SUFBC 2 D remains elusive. In this study, we report that the transcription of SUFB responds rapidly to leaf senescence, whereas the transcription of SUFC and SUFD does not. Intriguingly, their protein contents remain stable during leaf senescence. We further found that leaf death was occurred only when SUFB RNAi was induced, and SUFB and SUFC contents decreased much faster in the SUFB -RNAi lines than in the SUFC -RNAi lines, indicating that SUFB had a faster turnover rate than SUFC. Moreover, overexpressing SUFB increased the contents of SUFC and SUFD, and SUFBC 2 D, whereas overexpressing SUFC did not increase SUFB and SUFD. Our findings reveal that SUFB stabilizes SUFC and SUFD via forming SUFBC 2 D, whereas SUFC lacks this function. Furthermore, SUFB expression was sharply downregulated when the plants were subjected to iron deficiency, whereas SUFC and SUFD expression was not. Interestingly, the contents of all three SUF members decreased, indicating that plants degrade SUFBC 2 D in response to iron deficiency by downregulating SUFB transcription. We subsequently studied the degradation mechanism of SUFBC 2 D. Our results indicated that all SUFs are substrates of the caseinolytic protease (CLP) because they all accumulated in the CLP impaired mutant, and they physically interact with CLPS1, the substrate recognition adaptor of CLP. Collectively, our findings provide novel insights into how plants regulate SUFBC 2 D complex via SUFB to adapt to leaf senescence and iron deficiency.