M. Trabucchi et al., Molecular cloning of the cDNAs and distribution of the mRNAs encoding two somatostatin precursors in the African lungfish Protopterus annectens, J COMP NEUR, 410(4), 1999, pp. 643-652
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.