RNA-Protein Interactions in Light
Regulated Translation in Plants
Translation of chloroplast mRNAs is regulated in response to developmental
and environmental signals, including light. Biochemical and genetic
analysis has revealed that RNA binding proteins are required for
this translational regulation, and that these proteins interact
with RNA elements contained within the 5' untranslated region (UTR)
of chloroplast mRNAs to facilitate association of mRNAs with ribosomes
during translation initiation (Mayfield, et al., 1994).
Using molecular genetic analysis we have determined that several
RNA elements found in 5' UTR of the chloroplast mRNAs, including
Shine-Dalgarno sequences (ribosome binding site), are required for
protein binding and light-activated translation of many chloroplast
mRNAs. Alteration of the Shine-Dalgarno sequence in the 16S rRNA
resulted in the loss of translation of membrane associated proteins,
but had limited impact on translation of soluble chloroplast proteins,
suggesting that Shine-Dalgarno interactions may be used to differentially
translate mRNAs in chloroplasts.
We have biochemically isolated a number of proteins that bind
with high affinity and specificity to the 5' UTR of chloroplast
mRNAs, and have identified that binding of these proteins to the
mRNAs is required for translation initiation (Danon
and Mayfield, 1991). Binding activity for several of these proteins
is light-activated, which can be mimicked in vitro in a redox dependent
manner. These data suggest that light activated translation may
involve activation of protein binding to chloroplast mRNAs through
changes in the light generated redox potential of the cell (Danon
and Mayfield, 1994).
Model for translation of chloroplast mRNAs
The results from our lab and others has allowed us to propose a
model, although still incomplete, for the translation of chloroplast
mRNAs. Using the psbA mRNA as an example we propose that a
set of nuclear encoded RNA binding proteins are imported into the
chloroplast where they are activated to bind to specific chloroplast
mRNAs. Activation is achieved by changes in redox potential brought
about by generation of reducing equivalents from photosynthesis (1).
Binding of these proteins to the 5' UTR of the mRNA allows for association
of the mRNA with ribosomes (2), perhaps by interacting directly with
the unique domains of the small subunit or perhaps by altering the
structure of the 5' UTR to allow interaction with the ribosome (3).
As accumulation of the endogenous D1 protein increases photosynthetic
competence increases, and thus the generation of reducing potential
increases (4). Increased reducing potential increases translation
of the psbA mRNA. To keep synthesis from rising out of balance excess
D1 protein attenuates translation of its own mRNA via interactions
with the psbA 5' UTR, perhaps directly or perhaps via trans-acting
factors (5). Balance between light-activation and auto-attenuation
maintains a balanced supply of D1. There are a number of unaddressed
questions in this model, most notably how the unique ribosomal proteins
interact with chloroplast mRNAs, and how ribosomal proteins and mRNAs
interact with trans-acting protein factors. The nature of D1 auto-attenuation
and how D1 interacts with the psbA 5' UTR is also not yet understood.
Finally, the nature of RNA elements in 5' UTRs and how a 90 nucleotide
element accommodates both auto-attenuation and light-activated translation
remain large undefined. We are presently examining many of these questions
in the lab.
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