Nonessential Plastid-Encoded Ribosomal Proteins in Tobacco: A Developmental Role for Plastid Translation and Implications for Reductive Genome Evolution

Tobias T Fleischmann,Ralph Bock,Lars B Scharff,Sebastian Hasdorf,Sibah Alkatib,Mark A Schöttler

doi:10.1105/tpc.111.088906

Tobias T Fleischmann, Ralph Bock + Show 4 more

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https://doi.org/10.1105/tpc.111.088906

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Abstract

Plastid genomes of higher plants contain a conserved set of ribosomal protein genes. Although plastid translational activity is essential for cell survival in tobacco (Nicotiana tabacum), individual plastid ribosomal proteins can be nonessential. Candidates for nonessential plastid ribosomal proteins are ribosomal proteins identified as nonessential in bacteria and those whose genes were lost from the highly reduced plastid genomes of nonphotosynthetic plastid-bearing lineages (parasitic plants, apicomplexan protozoa). Here we report the reverse genetic analysis of seven plastid-encoded ribosomal proteins that meet these criteria. We have introduced knockout alleles for the corresponding genes into the tobacco plastid genome. Five of the targeted genes (ribosomal protein of the large subunit22 [rpl22], rpl23, rpl32, ribosomal protein of the small subunit3 [rps3], and rps16) were shown to be essential even under heterotrophic conditions, despite their loss in at least some parasitic plastid-bearing lineages. This suggests that nonphotosynthetic plastids show elevated rates of gene transfer to the nuclear genome. Knockout of two ribosomal protein genes, rps15 and rpl36, yielded homoplasmic transplastomic mutants, thus indicating nonessentiality. Whereas Δrps15 plants showed only a mild phenotype, Δrpl36 plants were severely impaired in photosynthesis and growth and, moreover, displayed greatly altered leaf morphology. This finding provides strong genetic evidence that chloroplast translational activity influences leaf development, presumably via a retrograde signaling pathway.

Highlights

Translation in plastids, such as chloroplasts, occurs on bacterialtype 70S ribosomes that are similar in structure and composition to bacterial ribosomes (Yamaguchi and Subramanian, 2000; Yamaguchi et al, 2000; Manuell et al, 2007)
It was found to be the most reduced plastid genome discovered to date in a seed plant, and its ribosomal protein gene content was considered for the identification of potentially nonessential genes
Using the sequence information from these reduced plastomes and the information on essential and nonessential genes in the model bacterium E. coli (Baba et al, 2006), the following plastid ribosomal protein genes were identified as potentially nonessential: rpl22, rpl23, rpl32, rpl36, rps3, rps15, and rps16 (Table 1)

Summary

Introduction

Translation in plastids, such as chloroplasts, occurs on bacterialtype 70S ribosomes that are similar in structure and composition to bacterial ribosomes (Yamaguchi and Subramanian, 2000; Yamaguchi et al, 2000; Manuell et al, 2007). All four RNA components (rRNAs) of chloroplast ribosomes are encoded by the plastid genome (plastome): the 23S, 5S, and 4.5S rRNAs of the large (50S) ribosomal subunit and the 16S rRNA of the small (30S) ribosomal subunit. The protein components of the plastid ribosome, the ribosomal proteins, are partly encoded in the nuclear genome. In the model plant tobacco (Nicotiana tabacum), 12 out of the 21 proteins of the small ribosomal subunit that have homologs in Escherichia coli are encoded in the chloroplast genome, whereas the remaining 9 proteins are nuclear encoded. Plastid ribosomes contain a small number of proteins that are not found in bacterial ribosomes, the plastid-specific ribosomal proteins. They are encoded by nuclear genes, and their functions in protein biosynthesis and/or ribosome assembly are still

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