Regulation of iron-sulfor cluster assembly in a facultative phototrophic Alpha-proteobacterium

Abstract

Fe-S proteins are present in all organisms and play many essential roles, but different kinds of stresses can destroy Fe-S cluster. R. sphaeroides is able to form photosynthetic complexes, which need more additional Fe-S cluster for the synthesis of the Cytbc1 complex and of bacteriochlorophyll. So R. sphaeroides had to develop some regulatory mechanisms to balance the damage and requirement of the assembly of Fe-S cluster.The isc-suf operon was identified as an important system for the regulation of Fe-S cluster assembly in R. sphaeroides. The results of this study show that three sense promoters contributed to the transcription of isc-suf (P1 and P2 together allowed higher transcription rates of isc-suf than P2 alone) and two anti-sense promoters influenced sense promoters transcription (P4 and P5 stimulated P2 activity). The iron-dependent regulators (Irr, IscR, Fur/Mur, RirA) and oxidative stress-dependent regulator (OxyR) play important roles in the expression of the isc-suf operon. The regulator IscR of R. sphaeroides plays a role as sensor for iron availability and organic peroxide stress (tBOOH) by bindings to P1, P2 and P3. The influences of Fur/Mur on the isc-suf operon were most likely indirect. Similarly, Irr influenced the P1 and P2 activities indirectly. However, direct binding of Irr to P3 was demonstrated. The activity of P5 was strongly increased in the OxyR mutant and the bindings of OxyR to P5 was demonstrated, indicating a repressing function of OxyR on P5 activity in R. sphaeroides. However, the two RirA homologues of R. sphaeroides play no major role in the oxidative stress and iron-dependent regulation of the isc-suf operon. These results demonstrate that a complex regulatory network of iron-sulfur cluster assembly is present in R. sphaeroides. The formation of photosynthetic complexes did not result in significant changes in the activities of the isc-suf promoters. The increase of the assembly of Fe-S cluster under low oxygen or anaerobic conditions was probably due to post-transcriptional regulatory mechanisms rather than the regulation of transcriptional level.