Molecular Biology and Evolution, Vol 13, 758-766, Copyright © 1996 by Society for Molecular Biology and Evolution
Y Courty, M Singer, I Rosinski-Chupin and F Rougeon
In rodents, the variable coding sequence (VCS) multigene family displays
extensive evolutionary divergence in the protein-coding region. While
certain VCS genes coding for proline-rich proteins (hPR- PB, mMSG1,
rPR-VB1) are conserved in primates and rodents, others seem to be specific
to certain genera. This appears to be the case for the Rattus genes forming
the A-subclass. This subclass is composed of three genes in R. norvegicus
and probably five genes in R. rattus. The first described VCSA gene (Rn.
VCSA1) was found to encode a prohormone-like protein named SMR1 (-VA1),
expressed mainly in the submandibular glands (SMG) of male rats. To further
understand the evolution of this variable multigene family, we have cloned
the two additional genes (Rn. VCSA2 and Rn. VCSA3) forming the R.
norvegicus A-subclass and three VCSA genes (Rr. VCSA1a, b and Rr. VCSA2) of
R. rattus. The putative SMR1 proteins encoded by all these genes display
the same prohormone- like structure as Rn. SMR1-VA1. However, we observe a
polymorphism in some internal cleavage sites which suggests that multiple
processing of the SMR1 proteins could result in the liberation of peptides
differing in structure and length. The phylogenetic analysis of the
sequences reveals that the duplication events giving rise to the VCSA1,
-A2, and - A3 progenitors were anterior to the R. norvegicus and R. rattus
split, and that a VCSA1 duplication event likely occurred specifically in
R. rattus. A striking observation is that the coding sequences of the VCSA
genes have rapidly diverged from their ancestors. Along all branches of the
phylogeny, the nonsynonymous divergence rate is identical or superior to
the synonymous divergence rate. We suggest that frequent changes in
functional requirements are mainly responsible for the episodic evolution
and the rapid divergence of the VCSA genes.
ORIGINAL ARTICLE
Episodic evolution and rapid divergence of members of the rat multigene family encoding the salivary prohormone-like protein SMR1
Unite de Genetique et Biochimie du Developpement, Institut Pasteur-URA CNRS, France.
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