Dosage compensation at the sgs4 locus of Drosophila melanogaster

The X-linked sgs4 gene of Drosophila melanogaster encodes a salivary glue protein. Non-dosage-compensated alleles of this gene have been described by G. Korge, in which males accumulate only about half of the Sgs4 polypeptide as do females. We show that the Sgs4 mRNA levels in dosage-compensated and non-dosage-compensated strains parallel the levels of Sgs4 polypeptide. Korge's genetic analysis of one of the non-dosage compensated alleles suggests that dosage compensation is controlled by sequences that lie 5' to the coding region. Sequences important for regulating the absolute levels of transcription of sgs4 have been identified as residing within a region of 600 base-pairs 5' to the site of transcription initiation. We have sequenced this region from two non-dosage-compensated strains and find no significant deviation from the sequence of dosage-compensated alleles. We therefore conclude that sequences that act to mediate the dosage compensation of sgs4 must lie outside this 600 base-pair region.