Homeobox Genes DataBase

Drosophila Genes in Development: Gap gene family

giant

Function

giant (gt) is a gap gene that codes for a transcriptional repressor. In keeping with its developmental group, lack of giant expression produces gaps in both anterior and posterior structures, specifically the labial and labral head structures and abdominal segments A5 through A7.

Giant helps define domains of expression for the pair-rule genes even-skipped, paired and fushi tarazu. It also delimits the anterior boundary of early Antennapedia expression.

Protein

The sequence of a cDNA from the Drosophila giant gene shows a basic domain followed by a leucine zipper motif. Both features contain characteristic conserved elements of the b-ZIP family of DNA-binding proteins (Capovilla M., 1992).

Subcellular location

Nuclear

Expression Pattern

Early in development this gene is expressed in two broad stripes, belting the embryo rather like a bikini, leaving bare the central "midriff" as well as the anterior and posterior ends.

giant expression in the head becomes increasingly complex over time, developing four stripes. Early on, one straddles the cephalic furrow and another enters the anterior stomodeal invagination to come into contact with a group of cells already strongly expressing giant inside the clypeolabrum. Mutation in both orthodenticleand empty spiracles will alter giant anterior expression. This suggests giant expression is modified by gap genes specific to the head. giant's complex expression pattern in the head suggests it is involved in as yet undocumented processes of head morphogenesis.

GT RNA is first detected quite early in development during nuclear cycle 12. One broad anterior stripe is formed 62 to 80% egg length (measuring from the posterior). A posterior domain is formed from 2 to 33% egg length. By the middle of nuclear cycle 13, the posterior domain has become more confined (15-20% egg length). During cellularization the anterior stripe resolves into two stripes, and at the end of cellulariation a new band of expression is present, located further toward the anterior, but not extending as far as the ventral most cells. As gastrulation begins, the anterior end of the ventral furrow reaches the original anterior stripe, and the cells expressing gt become internalized. The top of the initial anterior stripe straddles the cephalic furrow (Eldon E. D., 1991 and Kraut R., 1991a).

Dorso-lateral view of giant expression in a cleavage cycle 14 embryo undergoing cellularization. Anterior is to the left, and individual nuclei appear as small dots. This embryo was stained with antibodies to the Giant protein and then treated with fluorescently labeled second antibodies.