Pattern formation in the Drosophila embryo requires the concerted expression of maternal and zygotic genes. At least nineteen genes, twelve of which are maternally expressed, are involved in the establishment of dorsal-ventral polarity. Mutations in any one of these genes result in distinct alterations of cell fates and in the formation of an abnormal dorsal-ventral pattern. Mutants of the 'dorsal group', eleven of the maternal genes, have a common recessive phenotype similar to that described for dorsal, in that cells located at ventral and lateral positions assume dorsal fates and ventral structures fail to develop. Thus the dorsal group gene products may be involved in the establishment of a gradient of positional information along the dorsal-ventral axis. We have cloned snail (sna), a zygotic gene, whose expression is essential for the correct specification of dorsal-ventral pattern. In this report, we present evidence that the complementary DNA-deduced protein product of sna contains five copies of a nucleic acid-binding finger motif previously identified in two transcription factors, and in the protein product of several putative regulatory genes.

The Drosophila gene snail (sna) which encodes a zinc finger protein is essential for dorsal-ventral pattern formation in the developing embryo. We have defined a repertoire of SNAIL (SNA) binding sites using recombinant SNA proteins to select specific binding sequences from a pool of random sequence nucleotides. The bound sequences which were selected by multiple rounds of gel retardation and amplification by the polymerase chain reaction (PCR) were subsequently cloned and sequenced. The consensus sequence, 5'G/A A/t G/A A CAGGTG C/t A C 3', with a highly conserved core of 6 bases, CAGGTG, shares no significant homology with known binding sequences of other Drosophila zinc finger proteins. However, the CAGGTG core is identical to the core motif of aHLH (helix-loop-helix) binding sites. The strongest SNA binding is obtained with sequences containing this core motif whereas reduced binding is seen for sequences with canonical CANNTG HLH motifs. Interestingly, SNA binding is detected in the promoter region of the snail gene. Transient expression in co-transfection experiments using a SNA binding element (SBE) linked to a heterologous promoter indicates that SNA has the ability to function as a transcription activator.

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The zygotic effect gene snail (sna) encodes a zinc-finger protein required for mesoderm formation in Drosophila embryos. By in situ analysis, sna transcripts are first detected at syncytial blastoderm and persist until very late stages of embryogenesis. Expression of sna is transient and is observed in tissues derived from all three germ layers. Prior to germband elongation, sna RNA accumulation is consistent with its genetically determined role in mesoderm formation. Starting at germband elongation, a second phase of sna expression appears to be initiated, characterized by a highly dynamic accumulation of transcripts in the developing central and peripheral nervous systems. Translation of sna RNA is apparently delayed as the sna protein is not detected before the onset of gastrulation. Its regional distribution generally correlates with that of sna transcripts. The complex pattern of sna expression strongly suggests that the function of the gene is not restricted to mesoderm formation.