Figure 7.

A working model for the redox-mediated regulation of crhR expression. The model incorporates known elements from mustard chloroplasts and cyanobacteria. crhR expression is dynamically established by a complex interaction between redox-regulated transcription and mRNA stability. Reduction of electron carriers between Q_A in PSII and Q_O in cyt b₆f in the thylakoid membrane can either be accomplished naturally by light harvesting or by the metabolism of exogenously supplied Glc. Data presented here implicate interaction of plastoquinol with cyt b₆f as the environmental sensor that rapidly activates crhR expression at the transcriptional level, potentially by signaling through the signal transduction pathway known to initiate photosystem state transitions. Reduced plastoquinone has been reported to regulate directly the transcription of two chloroplast-encoded genes whose protein products are required to maintain photosynthetic electron flow. Accumulation of crhR mRNA is also regulated by a redox-regulated, but Glc metabolism-independent, mechanism. Redox regulation of transcription and mRNA stability provides a mechanism by which the cells could rapidly respond to alterations in redox conditions. The specificity of the redox regulation of crhR expression raises the intriguing possibility that CrhR-catalyzed RNA unwinding may be required for the efficient translation of its own and possibly other redox-regulated mRNAs.