Genetic dissection of pointed, a Drosophila gene encoding two ETS-related proteins.

The Drosophila gene pointed (pnt) is required for the differentiation of a number of tissues during embryogenesis, including the ventral ectoderm, the nervous system, the tracheal system and certain muscle fibers. The phenotypes associated with strong pointed alleles are reflected by a complex pointed expression pattern during embryogenesis. Two promoters, P1 and P2, separated by some 50 kb of genomic sequences, direct the transcription of two different transcript forms, encoding two different proteins related to the ETS family of transcription factors. To assess the individual functions of the two different pointed protein forms, we have generated new pointed alleles affecting either the P1 or the P2 transcript, termed P1 and P2 alleles, respectively. Genetic analysis reveals partial heteroallelic complementation between certain pointed P1 and P2 alleles. Surviving trans-heterozygous flies have rough eyes, abnormal wings and halters, suggesting a requirement for pointed function during their imaginal disc development. Further genetic analysis demonstrates that expression of a given pointed P2 allele depends on trans-acting transcriptional regulatory sequences. We have identified two chromosomal domains with opposite regulatory effects on the transcriptional activity of the pointed P2 promoter, one trans-activates and the other trans-represses pointed P2 expression. By deletion mapping we were able to localize these control regions within the 5' region of the pointed P2 transcript.