Edited transcripts compete with unedited mRNAs for trans-acting editing factors in higher plant chloroplasts

Chloroplast RNA transcripts of vascular plants undergo C to U editing at ap proximately 30 sites, but there is no consensus sequence that identifies a C to be edited. Both sequences closely surrounding an edited C and unidenti fied site-specific trans-acting factors have been shown to be important for editing. The ability of an already edited transgenic sequence to bind and thus titrate a trans-acting editing factor was evaluated for two editing si tes, ndhF and rpoB site 2. The U-containing rpoB transcripts did not affect editing of the endogenous rpoB transcripts, likely because the comparable C-containing transcripts containing 27 nucleotides surrounding the edited C were only 20% edited, indicating a low affinity of a trans-factor for this length of edited sequence. Surprisingly, U-containing ndhF transgene trans cripts reduced endogenous ndhF transcript editing to the same degree as a C -containing transgene transcript. This indicates that the C target of editi ng is not a critical recognition feature for the site-specific trans-acting factor. (C) 2001 Elsevier Science B.V. All rights reserved.