SELECTIVE CONSTRAINTS ON THE ACTIVATION DOMAIN OF TRANSCRIPTION FACTOR PIT-1

The POU transcription factor Pit-1 activates members of the prolactin/ growth hormone gene family in specific endocrine cell types of the pit uitary gland. Although Pit-1 is structurally conserved among vertebrat e species, evolutionary changes in the pattern of Pit-1 RNA splicing h ave led to a notable ''contraction'' of the transactivation domain in the mammalian lineage, relative to Pit-1 in salmonid fish. By site-dir ected mutagenesis we demonstrate that two splice insertions in salmon Pit-1, called beta (29 aa) and gamma (33 aa), are critical for coopera tive activation of the salmon prolactin gene. Paradoxically, Pit-1-dep endent activation of the prolactin gene in rat is enhanced in the abse nce of the homologous beta-insert sequence. This apparent divergence i n the mechanism of activation of prolactin genes by Pit-1 is target ge ne specific, as activation of rat and salmon growth hormone genes by P it-1 splice variants is entirely conserved. Our data suggest that effi cient activation of the prolactin gene in the vertebrate pituitary has significantly constrained the pattern of splicing within the Pit-1 tr ansactivation domain. Rapid evolutionary divergence of prolactin gene function may have demanded changes in Pit-1/protein interactions to ac commodate new patterns of transcriptional control by developmental or physiological factors.