Maize Chromosomal HMGc

Abstract

The chromosomal high mobility group (HMG) pro- teins are small and abundant non-histone proteins com- mon to eukaryotes. We have purified the maize HMGc protein from immature kernels and characterized it by mass spectrometry and amino acid sequence analysis. HMGc could be resolved into two similar proteins by reversed phase chromatography. Cloning and charac- terization of the corresponding cDNAs revealed that they encode two closely related maize HMGc proteins, now termed HMGc1 and HMGc2. Their theoretical masses of 15,316 and 15,007 Da are >300 Da lower than the masses determined for the proteins purified from maize, indicating post-translational modifications of the proteins. Despite sequence similarity to maize HMGa (and previously described homologous proteins of other species) amino acid sequence alignments reveal that HMGc is in several conserved regions distinct from these proteins. Consequently, we have identified a novel type of plant protein containing an HMG box DNA bind- ing 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 struc- ture-specifically since they formed complexes with DNA minicircles at concentrations 100-fold lower than the concentrations required to form complexes with linear fragments of identical sequence. Furthermore, HMGc1 and HMGc2 can constrain negative superhelical turns in plasmid DNA.