Structural and functional properties of linker histones and high mobility group proteins in polytene chromosomes.

Variants of histone H1 and high mobility group (HMG) proteins and their genes in Dipteran insects are being studied in our laboratory and have revealed different properties of DNA binding and intrachromosomal distribution. One of the H1 variants of Chironomus is found only in a minority of polytene chromosome bands and differs from the other H1 proteins of the same organism by genomic organization and by an inserted structural motif, the KAPKAP repeat, that is present also in single H1 variants of other, evolutionarily remote organisms. NH2-terminal peptides containing the KAPKAP repeat were found in vitro to interact with DNA, whereas no DNA interaction was observed with the homologous peptide of another H1 variant that does not contain the inserted KAPKAP repeat. We assume that H1 variants containing the KAP motif may interact with a stretch of linker DNA and package chromatin more tightly than other H1 variants. A large series of antibodies directed against different sites in all regions of the H1 molecule is being applied in studying the sites of interaction of the H1 molecule with other molecules in interphase chromatin in terms of antibody epitope accessibility. A search for insect proteins that share properties of the mammalian HMG proteins resulted in isolation and sequencing of two different HMG1 proteins and an HMGI protein. The HMG1 protein of the midge, Chironomus tentans, show a differential distribution in chromosomes. The more abundant cHMG1a protein appears uniformly distributed, whereas the less abundant cHMG1b protein could be localized only in chromosomal puffs. This strongly indicates that these highly similar proteins have different functions in chromatin. The Chironomus HMGI protein and the intron/exon organization of its gene were found to be very similar to human HMGI/Y proteins that are highly abundant in rapidly proliferating cells. Common properties of HMG1 and HMGI proteins include high affinity interaction with AT-rich DNA, irregular DNA structures, and the capacity to bend DNA. These properties suggest that the HMG proteins may have an architectural role in assembling different types of chromatin.