A Nucleus-Encoded Chloroplast Protein Regulated by Iron Availability Governs Expression of the Photosystem I Subunit PsaA inChlamydomonas reinhardtii 

Abstract

The biogenesis of the photosynthetic electron transfer chain in the thylakoid membranes requires the concerted expression of genes in the chloroplast and the nucleus. Chloroplast gene expression is subjected to anterograde control by a battery of nucleusencoded proteins that are imported in the chloroplast, where they mostly intervene at posttranscriptional steps. Using a new genetic screen, we identify a nuclear mutant that is required for expression of the PsaA subunit of photosystem I (PSI) in the chloroplast of Chlamydomonas reinhardtii. This mutant is affected in the stability and translation of psaA messenger RNA. The corresponding gene, TRANSLATION OF psaA1 (TAA1), encodes a large protein with two domains that are thought to mediate RNA binding: an array of octatricopeptide repeats (OPR) and an RNA-binding domain abundant in apicomplexans (RAP) domain. We show that as expected for its function, TAA1 is localized in the chloroplast. It was previously shown that when mixotrophic cultures of C. reinhardtii (which use both photosynthesis and mitochondrial respiration for growth) are shifted to conditions of iron limitation, there is a strong decrease in the accumulation of PSI and that this is rapidly reversed when iron is resupplied. Under these conditions, TAA1 protein is also down-regulated through a posttranscriptional mechanism and rapidly reaccumulates when iron is restored. These observations reveal a concerted regulation of PSI and of TAA1 in response to iron availability.

A Nucleus-Encoded Chloroplast Phosphoprotein Governs Expression of the Photosystem I Subunit PsaC in Chlamydomonas reinhardtii

“…Apart from Mbb1, the closest paralog of Mac1 mentioned above, these also include the TPR/HAT protein Nac2, which is required to stabilize psbD RNA (Kuchka et al, 1989;Nickelsen et al, 1999). Other examples are provided by members of the OPR family of helical-repeat proteins, such as Taa1 and Tab1, which are required for the stability and translation of psaA and psaB, respectively, and Mbi1, which is necessary for the stable accumulation of psbI mRNA (Lefebvre-Legendre et al, 2015;Wang et al, 2015). Also of interest is another OPR protein, Mcg1, which is required for stabilization of petG mRNA and generates a corresponding sRNA footprint at its 59end (Wang et al, 2015).…”

“…The iron that is released is bound by ferritin, which is upregulated at the level of translation (Busch et al, 2008). The response to iron deficiency also involves changes in the abundance of numerous other proteins and in particular of Taa1, which is degraded (Hohner et al, 2013; Lefebvre-Legendre et al, 2015). As previously mentioned, the latter is a nucleus-encoded protein of the OPR family that is specifically required for the stability and translation of psaA mRNA in the chloroplast.…”

“…Aliquots of the supernatant (800 mL) were mixed with 100 mL RNase buffer (100 mM Tris, pH 7.5, and 10 mM sodium acetate) with or without (mock treatment) 10 mg mL 21 RNaseA and incubated for 10 min at 22°C. The samples were loaded on 10-mL sucrose gradients (HKM buffer, 5 to 45% sucrose) as described previously (Lefebvre-Legendre et al, 2015). Twenty fractions (500 mL) were collected from the bottom.…”

“…Cell fractionation was described previously (Lefebvre-Legendre et al, 2015), using the protocol of Percoll gradient chloroplast isolation from Rivier et al (2001).…”

“…An early response of Chlamydomonas to iron deprivation is the downregulation of PSI and the remodeling of its light-harvesting antenna (Moseley et al, 2002;Naumann et al, 2005). There is a concomitant downregulation of Taa1, which is required for the stability and translation of psaA (Lefebvre-Legendre et al, 2015).…”