Interactions of recombinant HMGB proteins with branched RNA substrates

The high mobility group protein HMGB1 is a highly abundant chromosomal protein known to interact preferentially with DNA that is branched, bent or otherwise structurally altered. Biologically the protein is thought to facilitate promoter attachment by transcription factors. Recently, however, HMGB1 has been shown to have biological roles beyond that of an architectural DNA-binding protein. Here we investigate the binding interactions of recombinant HMGB1 proteins with two branched RNA's E. coli 5S rRNA and the group I intron ribozyme from Azoarcus pre-tRNA^Ile. Using competitive electrophoretic mobility and circular dichroism binding assays, we show that HMGB proteins bind both substrates with high affinity. We also report that a recombinant rat HMGB protein, rHMGB1b, inhibits RNA cleavage by the ribozyme. These results raise the possibility that HMGB proteins possess structure dependent RNA binding activity and can modulate RNA processing as well as transcription.