"We have reported that
mab-22 encoding a T-box containing transcription factor-TBX-2 is required to execute the ray assembly program. The
mab-22(
bx59) mutant exhibits an extensive ray missing phenotype, suggesting that
mab-22 is acting in all rays. However, the
mab-22::gfp reporter containing all necessary regulatory elements to rescue the
mab-22 mutant displays reporter activity only in the structural cells of rays 1, 5 and 7. Initially, we hypothesized that
mab-22 expression is under its own negative regulation in other rays. To test this hypothesis, the
mab-22::gfp reporter line was crossed into the
bx59 mutant. The incidence of expression and intensity of GFP signals in the structural cells of rays 1, 5 and 7 surged in such a genetic background. Besides, the reporter expression could also be detected in all rays.To refine this hypothesis of feedback regulation and test if it is mediated by direct binding of
mab-22 product onto sequences spanning the locus, we first identified the mouse TBX2 as the top MAB-22 ortholog with experimentally defined binding consensus sequences. We scanned through the
mab-22 genomic sequence on the reporter using the two defined mouse TBX2 binding consensus sequences. Two putative MAB-22 binding sites residing on intron 1 were identified. When these two binding sites were deleted altogether,
mab-22 reporter expression in transformed animals was detected in all ray structural cells, and at a higher level in rays 1, 5 and 7. Collectively, these results suggested that
mab-22 expression is under its own regulation. To the best of our knowledge, it is the first report that a T-box gene expression is modulated by a negative regulatory loop. Site-directed mutagenesis is in progress to evaluate the function of these two putative binding sites individually and to characterize the
mab-22 expression control mechanism in more detail. (This study is supported by the Research Grants Council, Hong Kong)."