Published online 26 January 2004
Nucleic Acids Research, 2004, Vol. 32, No. 2 542-550
© 2004 Oxford University Press
Surprising features of plastid ndhD transcripts: addition of non-encoded nucleotides and polysome association of mRNAs with an unedited start codon
Westfälische Wilhelms-Universität Münster, Institut für Biochemie und Biotechnologie der Pflanzen, Hindenburgplatz 55, D-48143 Münster, Germany
*To whom correspondence should be addressed. Tel: +49 251 83 24790/91; Fax: +49 251 83 28371; Email: rbock{at}uni-muenster.de
Present address:
Aitor Zandueta-Criado, Instituto de Agrobiotecnología y Recursos Naturales, Universidad Pública de Navarra/Consejo Superior de Investigaciones Científicas, Ctra. de Mutilva s/n, Mutilva Baja 31192 Navarra, Spain
RNA editing in higher plant plastids is a post- transcriptional RNA maturation process changing single cytidine nucleotides into uridine. In the ndhD transcript of tobacco and several other plant species, editing of an ACG codon to a standard AUG initiator codon is believed to be a prerequisite for translation. In order to test this assumption experimentally, we have analyzed the editing status of ndhD mRNA species in the process of translation. We show that unedited ndhD transcripts are also associated with polysomes in vivo, suggesting that they are translated. This surprising finding challenges the view that ACG to AUG editing is strictly required to make the ndhD message translatable and raises the possibility that ACG can be utilized as an initiator codon in chloroplasts. In addition, we have mapped the termini of the ndhD transcript and discovered a novel form of RNA processing. Unexpectedly, we find that highly specific sequences are added to the 3' end of the ndhD mRNA at high frequency. We propose a model in which these sequences are added by the successive action of a CCA-adding enzyme (tRNA nucleotidyltransferase) and an RNA-dependent RNA polymerase (RdRp) activity. The presence of an RdRp activity may have general implications also for other steps in plastid gene expression.
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