The chloroplast ycf3 and ycf4 open reading frames of Chlamydomonas reinhardtii are required for the accumulation of the photosystem I complex

Abstract

The chloroplast genes ycf3 and ycf4 from the green alga Chlamydomonas reinhardtii have been characterized. The deduced amino acid sequences of Ycf4 (197 residues) and Ycf3 (172 residues) display 41-52% and 64-78% sequence identity, respectively, with their homologues from algae, land plants and cyanobacteria. In C. reinhardtii, ycf4 and ycf3 are co-transcribed as members of the rps9-ycf4-ycf3-rps18 polycistronic transcriptional unit into RNAs of 8.0 kb and 3.0 kb corresponding to the entire unit and to rps9-ycf4-ycf3, respectively. Using biolistic transformation, ycf4 and ycf3 were disrupted with a chloroplast selectable marker cassette. Transformants lacking ycf4 or ycf3 were unable to grow photoautotrophically and were deficient in photosystem I activity. Western blot analysis showed that the photosystem I (PSI) complex does not accumulate stably in thylakoid membranes of these transformants. Ycf4 and Ycf3 were localized on thylakoid membranes but not stably associated with the PSI complex and accumulated to wild-type levels in mutants lacking PSI. RNA blot hybridizations showed that transcripts of psaA, psaB and psaC accumulate normally in these mutants and use of chimeric reporter genes revealed that Ycf3 is not required for initiation of translation of psaA and psaB mRNA. Our results indicate that Ycf3 and Ycf4 are required for stable accumulation of the PSI complex.

An RNA recognition motif-containing protein is required for plastid RNA editing in Arabidopsis and maize

“…The failure to edit petB-668 could account for the severe PetD deficiency, as this editing event is essential for the accumulation of the cytochrome b 6 f complex (26). The reduction in ycf3-185 and ycf3-44 editing is likely to account for the loss of PsaD, as Ycf3 is required for the assembly and accumulation of photosystem I (27,28). The minor loss of other proteins likely results from compromised chloroplast transcription and translation, possibly because of defects in the editing of rpoB, rpl20, and rps8 (encoding subunits of the plastid RNA polymerase, large ribosomal subunit, and small ribosomal subunit, respectively).…”

APO1 Promotes the Splicing of Chloroplast Group II Introns and Harbors a Plant-Specific Zinc-Dependent RNA Binding Domain

“…However, the expression of the chloroplast ycf3 gene, which is required for the assembly and accumulation of PSI (Boudreau et al, 1997;Ruf et al, 1997), had not been investigated in apo1 mutants. The ycf3 gene in Arabidopsis, maize, and most other angiosperms includes two group II introns (ycf3-int1 and ycf3-int2); CAF1, with which Zm-APO1 coimmunoprecipitated, is required for the splicing of one of these introns (ycf3-int1).…”

“…The ycf3-int2 splicing defect was observed also in a second mutant allele of APO1 (apo1-2 in Figure 1C), confirming that the splicing defect is due to the disruption of APO1. Given that deletion of ycf3 results in the loss of PSI (Boudreau et al, 1997;Ruf et al, 1997;Landau et al, 2009), this splicing defect is sufficient to account for the absence of PSI in apo1 mutants.…”

Characterization of Two Photosynthetic Mutants of Maize1

“…Assembly involves the expression of chloroplast and nuclear genes, the targeting of subunits to their proper locations within the chloroplast, and the proper organization and assembly of the subunits and cofactors into functional complexes (Boudreau et al, 1997). A disruption at any one of these stages can have a detrimental effect on the whole organism.…”

“…Other genes have been characterized that affect the stability of the PSI complex: btpA (Bartsevich and Pakrasi, 1997), ycf3 and ycf4 (Boudreau et al, 1997), and pmgA (Hihara et al, 1998). These lesions specifically affect the concentration of the PSI complex, with little effect on other photosystems, and therefore may not be a likely explanation for the mutant hcf47 phenotype.…”