Cloning and characterization of the segment polarity gene cubitus interruptus Dominant of Drosophila

The segment polarity mutation, cubitus interruptus Dominant (ci^D), of Drosophila melanogaster causes defects in the posterior half of every embryonic segment. We cloned sequences from the ci^D region on the proximal fourth chromosome by "tagging" the gene with the transposable element P. Genetic and molecular evidence indicates that the P-element insertions, which all occurred within the same restriction fragment, are in 5′-regulatory regions of the ci^D gene within 3 kb of the first exon of its transcript. The putative ci^D transcript was identified on the basis of its absence in homozygous ci^D embryos. Its spatial pattern of expression during development is unusual in that, unlike most other segmentation genes, it exhibits uniform expression throughout cellular blastoderm and gastrulation and does not resolve into a periodic pattern until the end of the fast phase of germ-band elongation when it is present in 15 broad segmentally repeating stripes along the anterior-posterior axis of the embryo. Registration of the ci^D stripes of expression relative to the stripes of other segment polarity genes shows that ci^D is expressed in the anterior three-quarters of every segment. This registration does not correlate with the pattern defects observed in ci^D mutants. Sequence analysis indicates that the protein encoded by the ci^D transcript contains a domain of five tandem amino acid repeats that have sequence similarity to the zinc-finger repeats of the Xenopus transcription factor TFIIIA and that share the highest degree of identity with the human zinc-finger protein GLI, which has been found to be amplified in several human glioblastomas.