The Plastid Genome-Encoded Ycf4 Protein Functions as a Nonessential Assembly Factor for Photosystem I in Higher Plants  

Abstract

Photosystem biogenesis in the thylakoid membrane is a highly complicated process that requires the coordinated assembly of nucleus-encoded and chloroplast-encoded protein subunits as well as the insertion of hundreds of cofactors, such as chromophores (chlorophylls, carotenoids) and iron-sulfur clusters. The molecular details of the assembly process and the identity and functions of the auxiliary factors involved in it are only poorly understood. In this work, we have characterized the chloroplast genome-encoded ycf4 (for hypothetical chloroplast reading frame no. 4) gene, previously shown to encode a protein involved in photosystem I (PSI) biogenesis in the unicellular green alga Chlamydomonas reinhardtii. Using stable transformation of the chloroplast genome, we have generated ycf4 knockout plants in the higher plant tobacco (Nicotiana tabacum). Although these mutants are severely affected in their photosynthetic performance, they are capable of photoautotrophic growth, demonstrating that, different from Chlamydomonas, the ycf4 gene product is not essential for photosynthesis. We further show that ycf4 knockout plants are specifically deficient in PSI accumulation. Unaltered expression of plastid-encoded PSI genes and biochemical analyses suggest a posttranslational action of the Ycf4 protein in the PSI assembly process. With increasing leaf age, the contents of Ycf4 and Y3IP1, another auxiliary factor involved in PSI assembly, decrease strongly, whereas PSI contents remain constant, suggesting that PSI is highly stable and that its biogenesis is restricted to young leaves.

Proteomic Analysis of Chloroplast-to-Chromoplast Transition in Tomato Reveals Metabolic Shifts Coupled with Disrupted Thylakoid Biogenesis Machinery and Elevated Energy-Production Components

“…However, unlike its role in algae, in higher plants it is not essential. The knockdown of the YCF4 gene in tobacco (Nicotiana tabacum) results in an only partial reduction of the amount of PSI complex, indicating that other proteins can replace YCF4 (Krech et al, 2012). Similarly to the decline in the YCF4 protein during the transition from chloroplast to chromoplast observed here in tomato fruit, YCF4 declines continuously in old leaves of tobacco concomitantly with the decrease of photosynthetic activity (Krech et al, 2012).…”

“…A plastid genome-encoded protein, YCF4 (Solyc01g007360) belonging to the hypothetical chloroplast reading frame family (YCF) decreases strongly between the B and R stages in parallel with the late disassembly of PSI. The Ycf4 protein participates in the assembly of PSI in the alga Chlamydomonas reinhardtii (Onishi and Takahashi, 2009) and in higher plants (Krech et al, 2012). However, unlike its role in algae, in higher plants it is not essential.…”

The Arabidopsis nox Mutant Lacking Carotene Hydroxylase Activity Reveals a Critical Role for Xanthophylls in Photosystem I Biogenesis

“…] type, or even higher ( Figure 5B). Moreover, the amount of Ycf3 and Ycf4, assembly factors for PSI (Boudreau et al, 1997;Krech et al, 2012), was similar in both genotypes (see Supplemental Figure 6 online).…”

A Thylakoid Membrane Protein Harboring a DnaJ-type Zinc Finger Domain Is Required for Photosystem I Accumulation in Plants

“…Proteins that promote PSI assembly can be divided in two functional classes: (i) proteins involved in cofactor synthesis or attachment and (ii) proteins that assist the assembly of the complex by interacting with PSI subunits (3). Two chloroplast-encoded proteins (Ycf3 and Ycf4) and three nucleus-encoded proteins (Y3IP1, Pyg7/Ycf37, and PPD1) have been shown previously to fall into the latter category (4,5,(7)(8)(9)54). All of these proteins interact with subunits of the core PSI complex (3).…”