The Bithorax Complex of Drosophila melanogaster: Underreplication and morphology in polytene chromosomes

The level of polyteny of the Drosophila salivary gland chromosomes was dete rmined throughout the chromosome region 89E1-4, the locus of the Bithorax C omplex. A zone of underreplication spans the 300 kb of DNA from the Ubx to Abd-B loci. From the centromere proximal end of the complex, a 70-kb-long g radual decrease of polytenization starts with the Ubx transcription unit an d, after a floor corresponding to the abd-A locus, raises gradually back to the maximum over 70 kb in the region of the Abd-B transcription unit. In f lies carrying the mutation Suppressor of DNA Underreplication [Su(UR)ES], t he underreplication of the Bithorax Complex is fully suppressed, In the wil d type, the Bithorax Complex forms a weak paint featuring thinner bands sep arated by clefts or constrictions. In SU(UR)ES strain in contrast the 89E1- 4 band looks like a single solid band consisting of homogenous dense materi al, We speculate that the wild-type SU(UR)ES protein hampers DNA replicatio n of silenced domains and leads to their underreplication in salivary gland polytene chromosomes.