THE HISTONE H1 GENES OF THE DIPTERAN INSECT, CHIRONOMUS-THUMMI, FALL UNDER 2 DIVERGENT CLASSES AND ENCODE PROTEINS WITH DISTINCT INTRANUCLEAR DISTRIBUTION AND POTENTIALLY DIFFERENT FUNCTIONS

Four histone H1 genes of the midge, Chironomas thummi piger; and three H genes of the subspecies C. thummi thummi have been cloned and assig ned to the four different H1 proteins from C. tjlrrrtlllri larvae. Tog ether with an earlier cloned WI gene from C, thummi thummi [Hankeln, T . & Schmidt, E. R. (1991) Chromosoma 101, 25-31], these genes probably constitute the complete complement of H1 genes in both subspecies, Th e, were found to fall under two classes that differ remarkably in thei r gene cop!: numbers, genomic organization, structure of flanking sequ ences, codon usage, and expression during embryonic development, and t hat encode H1 proteins of divergent structure. Histone HI I-1 contains an inserted sequence, KAPKAPKAPKSPKAE in C, thummi piger, and KAPKAPK SPKAE in C. thummi thummi. that is lacking in the other H1 variants, H 1 II-I, H1 II-2, and WI III-I, In the immediate neighbourhood to the I nserted sequence, a substitution in tile H1 I-1 protein sequence drama tically enhances the potential to form a reversed turn. In early devel opment, H1 I-1 is expressed at a higher rate than the other H1 genes, The transcripts have a size of about 1 Iib; in addition, the HI I-1 ge ne exhibited two minor transcripts of about 2.5 and >3 kb size in midd le blastoderm that are possibly polyadenylated, Together with our earl ier finding that histone H1 I-1 is found in a limited number of polyte ne chromosome bands whereas the other H1 histones are uniformly distri buted in chromatin, these results intimate functional differences betw een the two classes of H1 genes and their products.