The knirps and knirps-related genes organize development of the second wing vein in Drosophila.

The neighboring homologous knirps (kni) and knirps-related (knrl) genes in Drosophila encode transcription factors in the steroid hormone receptor superfamily. During early embryogenesis, kni functions as a gap gene to control expression of segmentation genes within the abdominal region of the embryo. In this study, we present evidence that kni and knrl link A/P positional information in larval wing imaginal discs to morphogenesis of the second longitudinal wing vein (L2). We show that kni and knrl are expressed in similar narrow stripes corresponding to the position of the L2 primordium. The kni and knrl L2 stripes abut the anterior border of the broad central expression domain of the Dpp target gene spalt major (salm). We provide evidence that radius incompletus (ri), a well-known viable mutant lacking the L2 vein, is a regulatory mutant of the kni/knrl locus. In ri mutant wing discs, kni and knrl fail to be expressed in the L2 primordium. In addition, the positions of molecular breakpoints in the kni/knrl locus indicate that the ri function is provided by cis-acting sequences upstream of the kni transcription unit. Epistasis tests reveal that the kni/knrl locus functions downstream of spalt major (salm) and upstream of genes required to initiate vein-versus-intervein differentiation. Mis-expression experiments suggest that kni and knrl expressing cells inhibit neighboring cells from becoming vein cells. Finally, kni and knrl are likely to refine the L2 position by positively auto-regulating their own expression and by providing negative feedback to repress salm expression. We propose a model in which the combined activities of kni and knrl organize development of the L2 vein in the appropriate position.

Functional and conserved domains of the Drosophila transcription factor encoded by the segmentation gene knirps.

The Drosophila gap gene knirps (kni) is required for abdominal segmentation. It encodes a steroid/thyroid orphan receptor-type transcription factor which is distributed in a broad band of nuclei in the posterior region of the blastoderm. To identify essential domains of the kni protein (KNI), we cloned and sequenced the DNA encompassing the coding region of nine kni mutant alleles of different strength and kni-homologous genes of related insect species. We also examined in vitro-modified versions of KNI in various assay systems both in vitro and in tissue culture. The results show that KNI contains several functional domains which are arranged in a modular fashion. The N-terminal 185-amino-acid region which includes the DNA-binding domain and a functional nuclear location signal fails to provide kni activity to the embryo. However, a truncated KNI protein that contains additional 47 amino acids exerts rather strong kni activity which is functionally defined by a weak kni mutant phenotype of the embryo. The additional 47-amino-acid stretch includes a transcriptional repressor domain which acts in the context of a heterologous DNA-binding domain of the yeast transcriptional activator GAL4. The different domains of KNI as defined by functional studies are conserved during insect evolution.

Gradients of Krüppel and knirps gene products direct pair-rule gene stripe patterning in the posterior region of the Drosophila embryo.

Abdominal segmentation of the Drosophila embryo requires the activities of the gap genes Krüppel (Kr), knirps (kni), and tailless (tll). They control the expression of the pair-rule gene hairy (h) by activating or repressing independent cis-acting units that generate individual stripes. Kr activates stripe 5 and represses stripe 6, kni activates stripe 6 and represses stripe 7, and tll activates stripe 7. Kr and kni proteins bind strongly to h control units that generate stripes in areas of low concentration of the respective gap gene products and weakly to those that generate stripes in areas of high gap gene expression. These results indicate that Kr and kni proteins form overlapping concentration gradients that generate the periodic pair-rule expression pattern.

[Pattern formation in Drosophila]. / Musterbildung bei Drosophila.

Drosophila proved an excellent system to study molecular processes in establishing the body pattern of an embryo. Genes which are active during oogenesis provide localized cues which regulate a cascade of zygotic genes that determines the developmental fate of the blastoderm cells along the longitudinal axis of the embryo.

Three hormone receptor-like Drosophila genes encode an identical DNA-binding finger.

The putative finger domain of knirps (kni), a member of the gap class of segmentation genes, was used to isolate two sequence-related genes of Drosophila melanogaster under reduced stringency hybridization conditions. The two kni homologous genes map close to kni in the proximal portion of the third chromosome. One of them is the previously identified gene knirps-related (knrl), kni and knrl are spatially co-regulated in both early and late stages of embryogenesis. Their posterior domains of expression at blastoderm stage are under the control of the maternal pattern organizer gene nanos. In contrast, the expression of the second kni homologous gene is restricted to the late embryonic gonads. Due to its site of expression, we termed this gene 'embryonic gonad' (egon). In addition to the conserved DNA-binding domain, these three genes share an additional sequence of 19 amino acids, the kni-box, adjacent to the finger region. The identical N-terminal Cys/Cys finger encoded by each of the three genes suggests that they code for DNA-binding proteins which might bind to similar (or even identical) target sequences.

Abdominal segmentation of the Drosophila embryo requires a hormone receptor-like protein encoded by the gap gene knirps.

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.

Molecular characterization of spalt, a homeotic gene required for head and tail development in the Drosophila embryo.

The isolation, identification and structure of the spalt gene is described. This novel homeotic gene of Drosophila is required for the establishment of the posterior-most head and the anterior-most tail segments of the embryo. It encodes a small mRNA of 0.8 kb which is under the control of over 15 kb of upstream sequences as indicated by the phenotype of transformed embryos. The putative spalt protein contains internal repeats and other interesting structural motifs but no homeo box. The spalt transcript accumulates motifs but no homeo box. The spalt transcript accumulates to high levels in the segmental anlagen affected in mutant embryos but is also found in regions of the embryo where no functional requirement has been demonstrated.

A conserved family of nuclear proteins containing structural elements of the finger protein encoded by Krüppel, a Drosophila segmentation gene.

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.