Identification of RNA Editing Sites in Chloroplast Transcripts from the Maternal and Paternal Progenitors of Tobacco (Nicotiana tabacum): Comparative Analysis Shows the Involvement of Distinct Trans-Factors for ndhB Editing

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Abstract
RNA editing alters genomic nucleotide sequences at the transcript level. In higher plant chloroplasts, C-to-U conversion is known to occur at around 30 specific sites. The tobacco cultivar Nicotiana tabacum is an amphidiploid derived from ancestors of N. sylvestris (maternal) and N. tomentosiformis (paternal). The chloroplast genome of N. tabacum is believed to originate from an ancestor of N. sylvestris. To study the evolution of RNA editing in higher plant chloroplasts, editing sites in the two likely progenitors have first been identified based on those found in N. tabacum. Altogether 34, 33, and 32 editing sites have been found in the chloroplast transcripts from N. tabacum, N. sylvestris, and N. tomentosiformis, respectively. Thirty-one sites are conserved among the three species, whereas remarkable differences are observed in the editing of ndhB and ndhD transcripts. Sites 7 and 8 in ndhB mRNAs are separated only by five nt, and both are edited in N. tabacum and N. sylvestris. However, site 8 is not edited in N. tomentosiformis, indicating that distinct trans-factors are involved in the two editing events. The first site in ndhD mRNAs is edited to produce an AUG start codon in N. sylvestris as well as in N. tabacum but not in N. tomentosiformis, suggesting that a distinct mechanism operates for the translational initiation of N. tomentosiformis ndhD mRNAs. Four to six sites are edited partially in green leaves. Some of these sites may represent evolutionary intermediates in the process of losing editing events.

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