A Conserved Mechanism Controls Translation of Rubisco Large Subunit in Different Photosynthetic Organisms

Abstract

We previously proposed a mechanism for control of Rubisco expression and assembly during oxidative stress in Chlamydomonas reinhardtii. The N terminus of the large subunit (LSU) comprises an RNA recognition motif (RRM) that is normally buried in the protein, but becomes exposed under oxidizing conditions when the glutathione pool shifts toward its oxidized form. Thus, de novo translation and assembly of Rubisco LSU stop with similar kinetics and the unpaired small subunit (SSU) is rapidly degraded. Here we show that the structure of the N-terminal domain is highly conserved throughout evolution, despite its relatively low sequence similarity. Furthermore, Rubisco from a broad evolutionary range of photosynthetic organisms binds RNA under oxidizing conditions, with dissociation constant values in the nanomolar range. In line with these observations, oxidative stress indeed causes a translational arrest in land plants as well as in Rhodospirillum rubrum, a purple bacterium that lacks the SSU. We highlight an evolutionary conserved element located within a-helix B, which is located in the center of the RRM and is also involved in the intramolecular interactions between two LSU chains. Thus, assembly masks the N terminus of the LSU hiding the RRM. When assembly is interrupted due to structural changes that occur under oxidizing conditions or in the absence of a dedicated chaperone, the N-terminal domain can become exposed, leading to the translational arrest of Rubisco LSU. Taken together, these results support a model by which LSU translation is governed by its dimerization. In the case that regulation of type I and type II Rubisco is conserved, the SSU does not appear to be directly involved in LSU translation.

Rubisco large-subunit translation is autoregulated in response to its assembly state in tobacco chloroplasts

“…Light stress generates reactive oxygen species, which can reversibly or irreversibly inactivate Rubisco. It was recently shown that methyl viologen-induced oxidative stress leads to inhibition of LS synthesis in tobacco and the purple bacterium Rhodospirillum rubrum (20), as previously shown for Chlamydomonas (34). Because Rhodospirillum lacks the SS, LS translational regulation must be SS-independent, as we have concluded is the case in tobacco.…”

“…Alternatively, repression may parallel bacterial ribosomal protein feedback regulation, where a direct interaction between the protein and its cognate RNA is observed (30). Interestingly, gel mobility shift assays reveal an LS N-terminal domain RNA-binding activity under oxidizing conditions, although this activity is nonspecific (20,31). The regulatory significance of this domain remains to be tested in vivo.…”

“…On the other hand, deletion of Chlamydomonas RBCS genes (17) or knocked-down expression using antisense technology in tobacco (18,19), showed that LS translation declines in absence of its assembly partner. LS synthesis is also subject to repression under oxidative stress (20). Nevertheless, the mechanism(s) leading to translational inhibition have not been solved.…”

Constraining the timing of the Great Oxidation Event within the Rubisco phylogenetic tree

“…It comprises an RNA recognition motif that becomes exposed under oxidizing conditions when the glutathione pool shifts toward its oxidized form (Cohen, Sapir, & Shapira, 2006). The oxygen sensitivity of the N‐terminus, and its role binding to RNA, may play an evolutionarily significant role in regulating Rubisco expression (Cohen et al., 2006; Kapralov & Filatov, 2007; Yosef et al., 2004). Regulation of expression can attenuate acute physiological response to perturbation inside or outside the cell (Bailey, 1991; Berry, Mure, & Yerramsetty, 2016).…”

“…For these reasons, it is possible that this substitutional enrichment associated with the N‐terminus may reflect an initial adaptive attempt to cope with increasing oxidative stress prior to the GOE. (Cohen et al., 2006). …”