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In plastids, editing of an ACG codon to an AUG codon creates the translation initiation codon for the psbL and ndhD transcripts in tobacco. To identify the RNA segment required for psbL editing, chimeric kanamycin resistance genes were constructed containing psbL deletion derivatives, and tested in vivo for editing in transgenic plants. We report here that a 22 nucleotide segment is sufficient to direct efficient psbL editing, including 16 nucleotides upstream and five nucleotides downstream of the editing site. Mutation of the A nucleotide to a C upstream of the editing site completely abolished editing, while mutation of the downstream G to a C only reduced the editing efficiency. Out of the 22 nucleotide editing target sequence, the 16 upstream nucleotides were found to compete with the endogenous psbL transcript for a depletable trans‐factor. To test whether editing of initiation codons involves a common trans‐factor, a chimeric gene containing the ndhD editing site was expressed in tobacco plastids. As for psbL, editing of the ndhD site requires a depletable trans‐factor. However, the ndhD trans‐factor is distinct from that required for psbL editing. Distinct cissequences and trans‐factor requirements for the psbL and ndhD editing sites indicate an individual recognition mechanism for the editing of plastid initiation codons.

In tobacco plastids, functional psbL mRNA is created by editing an ACG codon to an AUG translation initiation codon. To determine if editing may occur in a chimeric mRNA, the N‐terminal part of psbL containing the editing site was translationally fused with the aadA and kan bacterial genes. The chimeric constructs were introduced into the tobacco plastid genome by targeted gene insertion. Editing of the chimeric mRNAs indicated that the 98 nt fragment spanning the psbL editing site contains all cis information required for editing. Expression of the chimeric gene transcripts led to a significant decrease in the editing efficiency of the endogenous psbL mRNA. However, the efficiency of editing in the transplastomic lines was unchanged for four sites in the rpoB and ndhB mRNAs. Reduced efficiency of psbL editing, but not of the other four sites, in the transplastomic lines indicates depletion of psbL‐specific editing factor(s). This finding implicates the involvement of site‐specific factors in editing of plastid mRNAs in higher plants.

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