Heat Shock Protein HSP101 Binds to the Fed-1 Internal Light Regulatory Element and Mediates Its High Translational Activity

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Abstract
The internal light-regulatory element (iLRE) of ferredoxin (Fed-1) mRNA, comprising the 5′ leader and at least the first 13 codons of the open reading frame, controls transcript abundance after illumination of the plant in a translation-dependent manner. We have characterized the RNA binding activities associated with the Fed-1 iLRE and have identified one activity as the heat shock protein HSP101, a protein shown to bind the 5′ leader of tobacco mosaic virus. HSP101 was sufficient and necessary to mediate a high level of translational activity from a Fed-1 iLRE-containing mRNA in yeast. Moreover, the Fed-1 iLRE substantially enhanced translation of reporter mRNAs in plant protoplasts expressing HSP101. Expression of HSP101 was subject to developmental regulation in leaves in that expression was highest in young leaves. These data suggest that Fed-1 mRNA may use the HSP101 regulatory mechanism as a means of ensuring a high level of translation required for the light-mediated regulation of Fed-1 mRNA stability.