Heat shock protein HSP101 binds to the Fed-1 internal light regulatory element and mediates its high translational activity

The internal light-regulatory element (iLRE) of ferredoxin (Fed-1) mRNA, co mprising 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 activit ies 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. Moreo ver, the Fed-1 iLRE substantially enhanced translation of reporter mRNAs in plant protoplasts expressing HSP101, Expression of HSP101 was subject to d evelopmental regulation in leaves in that expression was highest in young l eaves. These data suggest that Fed-1 mRNA may use the HSP101 regulatory mec hanism as a means of ensuring a high level of translation required for the light-mediated regulation of Fed-1 mRNA stability.