Molecular cloning of the cDNAs and distribution of the mRNAs encoding two somatostatin precursors in the African lungfish Protopterus annectens

The occurrence of two somatostatin precursors, PSS1 and PSS2, yielding S-14 (SS1) and the variant [Pro(2), Met(13)]S-14 (SS2), has been recently repor ted in the frog Rana ridibunda. The evolutionary significance of frog PSS2 is unclear because its sequence exhibits very little similarity with other known vertebrate somatostatin precursors. In the present study, we report o n the characterization of two somatostatin precursor cDNAs from the brain o f the African lungfish Protopterus annectens. One of the cDNAs encodes a 11 5-amino-acid protein that contains the SS1 sequence at its C-terminal extre mity and thus is clearly homologous to PSS1. Comparison with other vertebra te PSS1 showed that lungfish PSS1 is more closely related to PSS1 fi om tet rapods than to PSS1 from fish. The other cDNAs encodes a 109-amino-acid pro tein that contains a somatostatin variant [Pro(2)]S-14 at its C-terminal ex tremity. Sequence analysis of this second precursor indicated that it is th e lungfish counterpart of frog PSS2. Northern blot analysis showed that lun gfish PSS1 mRNA is widely distributed in the central nervous system and in peripheral organs, including the pancreas and gastrointestinal tract. In co ntrast, PSS2 mRNA was primarily found in the central nervous system but not in the pancreas or gut. In situ hybridization studies showed that the two genes are differentially expressed in various regions of the lungfish brain . The present data indicate that the PSS2 gene, initially discovered in fro g, appeared early in vertebrate evolution, before the emergence of the tetr apod lineage. The recent isolation of a [Pro(2)]S-14 variant in the sturgeo n, whose sequence is identical to that of lungfish SS2, suggests that the P SS2 gene may actually be present in the genome of all Osteichthyii. (C) 199 9 Wiley-Liss, Inc.