Structures of cDNAs encoding chum salmon pituitary-specific transcription factor, Pit-1/GHF-1.

Pit-1/GHF-1, a POU family transcription factor, was originally isolated as a protein that contributes to the pituitary-specific expression of the mammalian growth hormone(GH)-encoding gene. Pit-1/GHF-1 has been recently implicated in the development of three types of hormone-producing cells which secrete GH, prolactin (PRL) and thyroid-stimulating hormone. A presumed pituitary hormone, somatolactin (SL) which belongs to the GH/PRL family, was previously isolated from fish. As the first step for determining the possible involvement of the Pit-1 protein in the pituitary-specific expression of the SL gene and/or in the development of SL-producing cells, chum salmon Pit-1 cDNA clones were isolated. Chum salmon Pit-1 mRNAs were mainly 2 kb and 3 kb in size, and specifically expressed in the pituitaries. They encoded a 365-amino acid (aa) protein, which was 74 aa larger than that found in mammals. The identity of aa sequences between salmon and rat Pit-1 was 69%. These proteins were highly conserved in the region specific for the POU family. Salmon Pit-1 had two additional sequences each consisting of about 30 aa in the N-terminal region. The sequence involved in transcriptional activation is considered to be located in the region including the above sequences, and thus, salmon Pit-1 may possibly have novel functions distinct from those of rat Pit-1.