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 thei r genes in Dipteran insects are being studied in our laboratory and ha ve revealed differential properties of DNA binding and intrachromosoma l 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 insert ed structural motif, the KAPKAP repeat, that is present also in single H1 variants of other, evolutionarily remote organisms. NH2-terminal p eptides containing the KAPKAP repeat were found in vitro to interact w ith DNA, whereas no DNA interaction was observed with the homologous p eptide of another H1 variant that does not contain the inserted KAPKAP repeat. We assume that H1 variants containing the KAP motif may inter act with a stretch of linker DNA and package chromatin more tightly th an other H1 variants. A large series of antibodies directed against di fferent sites in all regions of the H1 molecule is being applied in st udying the sites of interaction of the H1 molecule with other molecule s in interphase chromatin in terms of antibody epitope accessibility. A search for insect proteins that share properties of the mammalian HM G proteins resulted in isolation and sequencing of two different HMG1 proteins and an HMGI protein. The HMG1 proteins of the midge, Chironom us tentans, show a differential distribution in chromosomes. The more abundant cHMG1a protein appears uniformly distributed, whereas the les s abundant cHMG1b protein could be localized only in chromosomal puffs . This strongly indicates that these highly similar proteins have diff erent functions in chromatin. The Chironomus HMGI protein and the intr on/exon organization of its gene were found to be very similar to huma n HMGI/Y proteins that are highly abundant in rapidly proliferating ce lls. Common properties of HMG1 and HMGI proteins include high affinity interaction with AT-rich DNA, irregular DNA structures, and the capac ity to bend DNA. These properties suggest that the HMG proteins may ha ve an architectural role in assembling different types of chromatin.