The region-specific homeotic gene fork head (fkh) promotes terminal as opposed to segmental development in the Drosophila embryo. We have cloned the fkh region by chromosomal walking. P element-mediated germ-line transformation and sequence comparison of wild-type and mutant alleles identify the fkh gene within the cloned region. fkh is expressed in the early embryo in the two terminal domains that are homeotically transformed in fkh mutant embryos. The nuclear localization of the fkh protein suggests that fkh regulates the transcription of other, subordinate, genes. The fkh gene product, however, does not contain a known protein motif, such as the homeodomain or the zinc fingers, nor is it similar in sequence to any other known protein.

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Krüppel (Kr), a segmentation gene of Drosophila, encodes a protein sharing structural features of the DNA-binding "finger motif" of TFIIIA, a Xenopus transcription factor. Low-stringency hybridization of the Kr finger coding sequence revealed multiple copies of homologous DNA sequences in the genomes of Drosophila and other eukaryotes. Molecular analysis of one Kr-homologous DNA clone identified a developmentally regulated gene. Its product, a finger protein, relates to Kr by the invariant positioning of crucial amino acid residues within the finger repeats and by a stretch of seven amino acids connecting the finger loops, the "H/C link." This H/C link is conserved in several nuclear and chromosome-associated proteins of Drosophila and other eukaryotic organisms including mammals. Our results demonstrate a new subfamily of evolutionarily conserved nuclear and possibly DNA-binding proteins that again relate to a Drosophila segmentation gene as in the case of the homeo domain.

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The body pattern along the anterior-posterior axis of the insect embryo is thought to be established by two organizing centres localized at the ends of the egg. Genetic analysis of the polarity-organizing centres in Drosophila has identified three distinct classes of maternal effect genes that organize the anterior, posterior and terminal pattern elements of the embryo. The factors provided by these gene classes specify the patterns of expression of the segmentation genes at defined positions along the longitudinal axis of the embryo. The system responsible for organizing the posterior segment pattern is a group of at least seven maternal genes and the zygotic gap gene knirps (kni). Their mutant phenotype has adjacent segments in the abdominal region of the embryo deleted. Genetic analysis and cytoplasmic transplantation experiments suggested that these maternal genes are required to generate a 'posterior activity' that is thought to activate the expression of kni (reviewed in ref. 2). The molecular nature of the members of the posterior group is still unknown. Here we report the molecular characterization of the kni gene that codes for a member of the steroid/thyroid receptor superfamily of proteins which in vertebrates act as ligand-dependent DNA-binding transcription regulators.

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Krüppel is a member of the 'gap' class of segmentation genes of Drosophila melanogaster, mutations of which cause contiguous groups of segments of the fruitfly embryo to fail to develop. In the case of Krüppel mutant embryos, thoracic and anterior abdominal segments are deleted. The molecular cloning of the Krüppel locus will lead to an understanding of the crucial role that gap genes seem to have in early embryonic development. It has already allowed the identification of a blastoderm-specific Krüppel transcript and the phenotypic rescue of mutant embryos by injected cloned DNA.

Krüppel (Kr), a gap gene of Drosophila, shows complex spatial patterns of expression during the different stages of embryogenesis. In order to identify cis‐acting sequences required for normal Kr gene expression, we analysed the expression patterns of fusion gene constructs in transgenic embryos. In these constructs, bacterial lacZ expression was placed under the control of Kr sequences in front of a basal promoter. We identified cis‐acting Kr control units which drive beta‐galactosidase expression in 10 known locations of Kr expression in early and late embryos. More than one cis‐regulatory element drives the expression in the anterior domain at the blastoderm stage, in the nervous system, the midline precursor cells and in the amino‐serosa. In addition, two cis‐acting elements direct the first zygotic expression of Kr in a striped subpattern within the central region of the blastoderm embryo. Both elements respond to alterations in the activities of maternal organizer genes known to be required for Kr expression in establishing the thoracic and anterior abdominal segments in the wild‐type embryo.