MAIZE CHROMOSOMAL HMGC - 2 CLOSELY-RELATED STRUCTURE-SPECIFIC DNA-BINDING PROTEINS SPECIFY A 2ND TYPE OF PLANT HIGH-MOBILITY GROUP BOX PROTEIN

The chromosomal high mobility group (HMG) proteins are small and abund ant non-histone proteins common to eukaryotes. We have purified the ma ize HMGc protein from immature kernels and characterized it by mass sp ectrometry and amino acid sequence analysis. HMGc could be resolved in to two similar proteins by reversed phase chromatography. Cloning and characterization of the corresponding cDNAs revealed that they encode two closely related maize HMGc proteins, now termed HMGc1 and HMGc2. T heir theoretical masses of 15,316 and 15,007 Da are >300 Da lower than the masses determined for the proteins purified from maize, indicatin g post-translational modifications of the proteins. Despite sequence s imilarity to maize HMGa (and previously described homologous proteins of other species) amino acid sequence alignments reveal that HMGc is i n several conserved regions distinct from these proteins, Consequently , we have identified a novel type of plant protein containing an HMG b ox DNA binding domain and belonging to the HMG1 protein family. HMGc1 and HMGc2 were expressed in Escherichia coli, purified to homogeneity, and analyzed for their DNA binding properties. They proved to bind to DNA structure-specifically since they formed complexes with DNA minic ircles at concentrations similar to 100-fold lower than the concentrat ions required to form complexes with linear fragments of identical seq uence. Furthermore, HMGc1 and HMGc2 can constrain negative superhelica l turns in plasmid DNA.