High mobility group chromosomal proteins bind to AT-rich tracts flanking plant genes

AT-rich sequences in the 5' flanking regions of several plant genes have been shown to bind nuclear proteins, but the nature of these proteins has remained largely unknown. We report here that certain plant high mobility group (HMG) chromosomal proteins can interact specifically (in the presence of excess non-specific competitor) with AT-rich sequences located upstream of the pea ferredoxin 1 gene (Fed-1) and a member of the wheat Em gene family. Binding was observed with highly purified preparations of HMGa or HMGb, but not with HMGc or HMGd. HMG-DNA complexes were similar to one of the two types of Fed-1 complexes we observed previously using pea nuclear extracts [7]. HMG binding to the Fed-1 DNA was localized to a region containing AT-rich sequences; very similar sequences are present 5' to Em and several other plants genes. Such sequences have been shown to bind unidentified nuclear proteins in a number of these systems. Binding experiments with a synthetic oligo (dA).oligo (dT) probe and competition experiments with synthetic DNA polymers suggest that HMG binding may depend upon structural features of AT-rich DNA rather than being sequence-specific. We discuss the implications of these findings and suggest a role for HMG binding which is consistent with previous evidence linking HMGs with transcriptionally competent chromatin.