THE HETEROCHROMATIC ROLLED GENE OF DROSOPHILA-MELANOGASTER IS EXTENSIVELY POLYTENIZED AND TRANSCRIPTIONALLY ACTIVE IN THE SALIVARY-GLAND CHROMOCENTER

This paper reports a cytogenetic and molecular study of the structural and functional organization of the Drosophila melanogaster chromocent er. The relations between mitotic (constitutive) heterochromatin and a lpha- and beta-heterochromatin are not fully understood. In the presen t work, we have studied the polytenization of the rolled (rl) locus, a 100-kb genomic region that maps to the proximal heterochromatin of ch romosome 2 and has been previously thought to contribute to alpha-hete rochromatin. We show that rolled undergoes polytenization in salivary gland chromosomes to a degree comparable to that of euchromatic genes, despite its deep heterochromatic location. In contrast, both the Bari -1 sequences and the AAGAC satellite repeats, located respectively to the left and right of rl, are severely underrepresented and thus both appear to be alpha-heterochromatic. In addition, rue found that rl is transcribed in polytene tissues. Together, the results reported here i ndicate that functional sequences located within the proximal constitu tive heterochromatin can undergo polytenization, contributing to the f ormation of beta-heterochromatin. The implications of this finding to chromocenter structure are discussed.