Long human TSC22DF isoforms can replace BunA function in Drosophila. (a) Schematic drawing of human and Drosophila TSC22DF proteins that were tested for their ability to rescue the lethality of bun mutants. The long isoforms possess two short conserved stretches named motif 1 and motif 2. Whereas BunA represents the long TSC22DF isoforms in Drosophila, BunB and BunC are two of the short isoforms. (b) Expression of long TSC22DF isoforms restores the viability of bun mutants. The quality of the rescue is indicated as a percentage of the expected Mendelian ratio. The Gal4 driver lines are ordered according to the strength of ubiquitous expression they direct during development, with arm-Gal4 being the weakest and Act5C-Gal4 the strongest driver line. In each experimental cross, n ≥ 200 progeny flies were analyzed. Leaky expression, without Gal4; 1 c and 2 c, one or two copies of the respective UAS construct. The ZH-attP-86Fb integration site seems to mediate strong expression as the UAS-attB-bunA constructs (ORF and cDNA) do not need to be driven by a Gal4 line for rescue, in contrast to the UAS-bunA construct (cDNA) generated by standard P-element-mediated germline transformation (inserted non-site-specifically on chromosome III). Note that too high expression of long TSC22DF members is harmful to flies. In a wild-type background, Act5C-Gal4-directed expression (n ≥ 200) of TSC22D2 and of bunA ORF kills the animals (0% survival). Expression from the bunA cDNA construct produces few escapers (3%), whereas expression from the bunA cDNA P-element construct and of TSC22D4 results in semi-viability (14% and 69%, respectively). Only TSC22D1.1 can be expressed by Act5C-Gal4 without compromising survival (>80%). Thus, there appears to be an optimal range of long TSC22DF concentration for viability.