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J. Cell Biol.,
Volume 143, Number 5, November 30, 1998 1145-1153
Department of Cell Biology and The Skaggs Institute for Chemical Biology, The Scripps Research Institute, La Jolla, California
92037
The 5' untranslated region of the chloroplast
psbA mRNA, encoding the D1 protein, is processed in
Chlamydomonas reinhardtii. Processing occurs just
upstream of a consensus Shine-Dalgarno sequence and
results in the removal of 54 nucleotides from the 5'
terminus, including a stem-loop element identified previously as an important structure for D1 expression.
Examination of this processing event in C. reinhardtii
strains containing mutations within the chloroplast or
nuclear genomes that block psbA translation reveals a
correlation between processing and ribosome association. Mutations within the 5' untranslated region of the
psbA mRNA that disrupt the Shine-Dalgarno sequence, acting as a ribosome binding site, preclude
translation and prevent mRNA processing. Similarly, nuclear mutations that specifically affect synthesis of
the D1 protein specifically affect processing of the
psbA mRNA. In vitro, loss of the stem-loop element
does not prohibit the binding of a message-specific protein complex required for translational activation of
psbA upon illumination. These results are consistent with a hierarchical maturation pathway for chloroplast
messages, mediated by nuclear-encoded factors, that
integrates mRNA processing, message stability, ribosome association, and translation.
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