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The 3′-Untranslated Region of Chloroplast psbAmRNA Stabilizes Binding of Regulatory Proteins to the Leader of the Message*

The 5′-leader and 3′-tail of chloroplast mRNAs have been suggested to play a role in posttranscriptional regulation of expression of the message. The regulation is thought to be mediated, at least in part, by regulatory proteins that are encoded by the nuclear genome and targeted to the chloroplast where they interact with chloroplast mRNAs. Previous studies identified high affinity binding of the 5′-untranslated region (UTR) of the chloroplastpsbA mRNA by Chlamydomonas reinhardtiiproteins. Here we tested whether the 3′-UTR of psbAmRNA alone or linked in cis with the 5′-UTR of the mRNA affects the high affinity binding of the message in vitro. We did not detect high affinity binding that is unique to the 3′-UTR. However, we show that the cis-linked 3′-UTR increases the stability of the 5′-UTR binding complex. This effect could provide a means for translational discrimination against mRNAs that are incorrectly processed.