Evolutionary constraints on the plastid tRNA set decoding methionine and isoleucine

Area of Science:

• Plant Molecular Biology
• Genetics
• Evolutionary Biology

Background:

• Seed plant plastid genomes typically encode 30 tRNAs, utilizing wobble and superwobble mechanisms for codon recognition.
• Most codon boxes are read by one or two tRNA species, reflecting organellar genome reduction.
• A notable exception is the AUN codon box (methionine/isoleucine), usually decoded by four tRNA species in seed plants.

Purpose of the Study:

• To investigate the necessity of the four AUN-decoding tRNA genes in tobacco plastids.
• To test the hypothesis that fewer than four tRNA species could suffice for AUN box decoding, as suggested by gene loss in parasitic lineages.

Main Methods:

• Performed knockout experiments for all four AUN-decoding tRNA genes in tobacco (Nicotiana tabacum) plastids.
• Assessed plant viability and growth under both autotrophic and heterotrophic conditions.
• Conducted phylogenetic analysis of the four plastid tRNA genes, focusing on bacterial features involved in anticodon discrimination.

Main Results:

• All four AUN-decoding tRNA genes were found to be essential for tobacco plant survival under both autotrophic and heterotrophic growth.
• The study suggests that parasitic species losing these genes might rely on imported tRNAs for plastid function.
• Phylogenetic analysis highlighted conserved bacterial features crucial for discriminating between tRNA species with CAU anticodons.

Conclusions:

• The four tRNA genes decoding the AUN box are indispensable for plastid function in tobacco.
• Gene loss in parasitic lineages likely indicates an alternative strategy, such as tRNA import, rather than reduced functional requirements.
• Conserved features in these tRNAs point to ancient mechanisms for precise codon decoding.