Cloning of a proopiomelanocortin cDNA from the pituitary of the Australianlungfish, Neoceratodus forsteri: Analyzing trends in the organization of this prohormone precursor

The polypeptide hormone precursor, proopiomelanocortin (POMC), was cloned a nd sequenced from the pituitary of the Australian lungfish, Neoceratodus fo rsteri, the only surviving species of the oldest extant lineage of lungfish . The Australian lungfish POMC cDNA had an open reading frame that coded fo r a 255-amino acid precursor. A comparison of POMC sequences from the Austr alian lungfish and the African lungfish indicated that the deduced amino ac id sequences for ACTH, beta-MSH, and beta-endorphin were over 90% identical . Furthermore, within the open reading frames of the two lungfish POMCs, th ere was 84% identity at the nucleotide level. Although a gamma-MSH-like reg ion was detected in the Australian lungfish POMC cDNA, this sequence contai ned mutations that have been detected in the gamma-MSH sequences of some ra y-finned fish and are not found in the gamma-MSH sequence of the African lu ngfish or those of tetrapods. In addition, the sequence of beta-endorphin i n the two species of lungfish has amino acid motifs that are found in the b eta-endorphin sequences of cartilaginous fish and ray-finned fish but not i n tetrapods. However, maximum parsimony analysis of the entire POMC open re ading indicated that the lungfish POMC sequences form a clade with two amph ibian POMC sequences rather than with POMC sequences from ray-finned fish. This result is consistent with the accepted view that the sarcopterygians ( lungfishes and tetrapods) are a monophyletic assemblage. Analysis of rates of divergence for various POMC sequences indicate that point mutations are accumulating in the lungfish POMC sequences at a slower rate than in either amphibian or mammalian POMC sequences. The phylogenetic implications of th ese observations are discussed. (C) 1999 Academic Press.