Conditional repression of essential chloroplast genes: Evidence for new plastid signaling pathways

Abstract

The development of a repressible chloroplast gene expression system in Chlamydomonas reinhardtii has opened the door for studying the role of essential chloroplast genes. This approach has been used to analyze three chloroplast genes of this sort coding for the α subunit of RNA polymerase (rpoA), a ribosomal protein (rps12) and the catalytic subunit of the ATP-dependent ClpP protease (clpP1). Depletion of the three corresponding proteins leads to growth arrest and cell death. Shutdown of chloroplast transcription and translation increases the abundance of a set of plastid transcripts that includes mainly those involved in transcription, translation and proteolysis and reveals multiple regulatory feedback loops in the chloroplast gene circuitry. Depletion of ClpP profoundly affects plastid protein homeostasis and elicits an autophagy-like response with extensive cytoplasmic vacuolization of cells. It also triggers changes in chloroplast and nuclear gene expression resulting in increased abundance of chaperones, proteases, ubiquitin-related proteins and proteins involved in lipid trafficking and thylakoid biogenesis. These features are hallmarks of an unfolded protein response in the chloroplast and raise new questions on plastid protein homeostasis and plastid signaling. This article is part of a Special Issue entitled: Chloroplast Biogenesis.

Interference with Clp protease impairs carotenoid accumulation during tomato fruit ripening

“…Constitutive or induced down-regulation of Clp protease activity in chloroplasts has been previously shown to cause an enhanced accumulation of plastidial chaperones, most probably to deal with general protein folding stress caused by reduced protein degradation ( Nishimura and van Wijk, 2015 ; Rochaix and Ramundo, 2015 ; Llamas et al , 2017 ). OR or OR-like proteins do not overaccumulate in Clp-defective chloroplasts, perhaps because these chaperones do not have a relevant role in this plastid type.…”

GUN1, a Jack-Of-All-Trades in Chloroplast Protein Homeostasis and Signaling

“…Taken together, these findings suggest that disruption of protein homeostasis in organelles can be sensed and transduced to the nucleus to induce the expression of a specific set of factors responsible for promoting folding and monitoring protein quality control (Ramundo and Rochaix, 2014 ; Rochaix and Ramundo, 2015 ). After entering the higher plant chloroplast, these factors are able to interact with the nucleoid-associated GUN1 protein (Figure 2 ), which might therefore play a role in the cpUPR process.…”

“…Intriguingly, several GUN1 interactors appear to accumulate to higher levels upon induction of the unfolded protein response (UPR) in Chlamydomonas reinhardtii chloroplasts, which is triggered upon conditional repression of the catalytic subunit of Clp protease (ClpP1; Ramundo et al, 2013 ; Ramundo and Rochaix, 2014 ; Rochaix and Ramundo, 2015 ). This finding suggests the possible involvement of GUN1 in the UPR signaling pathway.…”

Proteomic analysis of chloroplasts from chlorophyll-deficient melon mutant

“…This indicates that the mutant continues to produce some Calvin cycle intermediates (i.e., ribulose-5-phosphate). These intermediate products in the Calvin cycle are precursors to nucleic acids, aromatic amino acids, and vitamin synthesis (Rochaix and Ramundo 2015).…”

“…Some studies have revealed a negative feedback system in the chloroplast. The cessation of chloroplast transcription or translation by inactivation of rps12 leads to a marked increase in the abundance of the transcripts of most RNA polymerase and ribosomal protein genes (Rochaix and Ramundo 2015). In the mutant, a negative feedback system might exist, which may respond to Chl deficiency.…”