Structural homology of the product of the Drosophila Krüppel gene with Xenopus transcription factor IIIA

Segmentation of the Drosophila embryo is established at around the blastoderm stage 1–3 and requires both maternal information in the egg cytoplasm 4,5 and expression of the zygotic genome. Zygotic genes involved in segmentation have been defined by mutations that affect the segmentai pattern of the embryo 6–10 , and these fall into several different classes, such as the 'gap' genes, mutations of which cause the loss of contiguous segments 6 . Krüppel (K r ) is an example of a Drosophila gap genes, strong K r mutant embryos lacking all thoracic and five anterior abdominal segments, with part of the remaining posterior abdominal segments being present as a mirror-image duplication (weaker alleles cause shorter deletions 11,12 ). K r has been cloned 12 and shown to encode a blas-toderm-gastrulation stage-specific transcript expressed in regions of the embryo affected by K r mutants 13 . We report here that the protein product of the K r gene predicted from DNA sequences is structurally homologous to the transcription factor TFIIIA which regulates 5S gene expression in Xenopus . Like TFIIIA, the predicted K r protein has an internally repetitious sequence, and as has been suggested for TFIIIA, the repeat units may form DNA-binding domains.