Proximity-dependent enhancement of Sgs-4 gene expression in D. melanogaster.

A weakly expressed allele of Sgs-4 obtained from the Kochi strain of D. melanogaster (Sgs-4K) increases 4-fold in levels of accumulated transcript when paired with a wild-type (Oregon-R) Sgs-4 allele (Sgs-4ORE) and increases 9-fold when paired with a duplication of the wild-type Sgs-4 allele Confluens (Sgs-4Co). There is no enhancement of expression when Sgs-4K is paired with the Sgs-4 null allele Ber-I (Sgs-4BER) or with another weakly expressed Sgs-4 allele, Hikone-R (Sgs-4HIK); there is minimal enhancement when chromosome pairing is disrupted near the Sgs-4 locus by rearrangement of the wild-type Sgs-4 allele. Cytological analysis of third instar salivary gland nuclei from Kochi/Confluens heterozygotes suggests that complete pairing is not essential for Sgs-4K enhancement. Interactions between homologous loci that could produce a trans enhancement effect are discussed and a hypothesis is formulated to test the significance of specific upstream sequences from both alleles in expression enhancement.