The role of Ftz-F1 in the regulation of chinook salmon gonadotropin II beta subunit gene

grantor: University of TorontoIn teleosts, pituitary gonadotropin II (GTHII) is the maturation hormone whose specificity is defined by its ß subunit (GTHIIß). Salmon GTHIIß (sGTHIIß) gene is differentially regulated, but the mechanism underlying the regulation remains poorly understood. The objective of my thesis is to identify transcription factors controlling the differential expression of sGTHIIß gene. Transient cotransfection analysis of sGTHIIß gene promoter revealed that a mouse homolog of 'Drosophila fushi tarazu' factor 1 (Ftz-F1), also known as steroidogenic factor 1 (SF-1), stimulated the promoter activity. SF-1 acted in synergy with the estradiol-17ß (E2)-activated estrogen receptor (ER) to dramatically enhance sGTHIIß promoter activity. We have identified a zebrafish Ftz-F1 (zFF1A) and a C-terminally truncated variant (zFF1B). These zFF1 cDNAs are encoded by a single gene locus, ' zff1', which is mapped to linkage group 22. The expression of ' zff1' in the reproductive axis is similar to that of SF-1 in mammals. While zFF1A functionally resembled SF-1, zFF1B was found to be transactivation-deficient and antagonized the action of zFF1A 'via' competitively binding to the target promoters. The sub-domains required for the transactivation of zFF1A were characterized. Relatively, the contribution of A/B region at N-terminus was minimal. In addition to the C-terminal activation function (AF-2), a novel AF-3 in the hinge region (whose activity was dispensable from AF-2) was delineated. While the Identity box (I-box) comprises a 'dimerization surface' for heterodimeric nuclear receptors (NRs), it was insufficient to contribute to a direct zFF1A/ER interaction. Instead, AF-2 required the I-box, especially a hydrophobic heptad motif (heptad 9), to be transcriptionally active. The requirement of both AF-2 and I-box for zFF1A transactivation is relevant to studying other NRs. The differential regulation of GTHIIß gene is complex, involving both transcription activators and repressors. The current study represents the first example illustrating that vertebrate Ftz-F1 is directly involved in the regulation of a GTH ß gene. Furthermore, we show a synergy between Ftz-F1 and ER that is specific to the developmental stage-dependent expression of sGTHIIß and likely accounts for the dramatic increase of salmon pituitary GTHII during the spawning season.Ph.D