Microcloning reveals ahighfrequency ofrepetitive sequences characteristic ofchromosome 4andthef3-heterochromatin of

Microdissection andmicrocloning oftheeu- chromatin-heterochromatin transition region oftheDrosophila melanogaster polytene Xchromosome andpart oftheeuchro- matin ofchromosome 4reveals that they share certain features characteristic of(3-heterochromatin, which ismorphologically defined astheloosely textured material atthebases ofsome polytene chromosome arms. Botharemosaics ofmanydifferent middle-repetitive DNAsequences interspersed with single-copy DNAsequences. Sixty percent ofcloned inserts derived from division 20andabout 40percent fromsubdivisions l9EFofthe Xchromosome harbor atleast onerepetitive DNAsequence in anaverage insert of4.5kilobases. Norepeats havesignificant cross-hybridization toanyoftheeleven satellite DNAs,ortothe clustered-scrambled sequences present inpDml.Therepetitive elements are, ingeneral, confined totheg3-heterochromatic regions ofpolytene chromosomes, butsomeareadjacent to nomadic elements. Chromosome 4,however, hassomerepeats spread throughout itsentire euchromatin. Thesedatahave implications forthestructure oftransition zones between eu- chromatin andheterochromatin ofmitotic chromosomes and alsoprovide amolecular basis forreexamining someofthe unusual classical properties ofchromosome 4.