Molecular Biology and Evolution, Vol 16, 397-404, Copyright © 1999 by Society for Molecular Biology and Evolution
PT Bosma, M Blazquez, MA Collins, JD Bishop, G Drouin, IG Priede, K Docherty and VL Trudeau
The evolution of chordate glutamic acid decarboxylase (GAD; EC 4.1.1.15), a
key enzyme in the central nervous system synthesizing the neurotransmitter
gamma-amino-butyric acid (GABA) from glutamate, was studied. Prior to this
study, molecular data of GAD had been restricted to mammals, which express
two distinct forms, GAD65 and GAD67. These are the products of separate
genes and probably are derived from a common ancestral GAD following gene
duplication at some point during vertebrate evolution. To enable a
comprehensive phylogenetic analysis, molecular information of GAD forms in
other vertebrate classes was essential. By reverse transcriptase-polymerase
chain reaction (RT-PCR), partial nucleotide sequences of GAD were cloned
from brains of zebra finch (Taeniopygia guttata), turtle (Trachemys
scripta), goldfish (Carassius auratus), zebrafish (Danio rerio), and
armoured grenadier (Coryphaenoides (Nematonurus) armatus, a deep-sea fish),
and from the cerebral ganglion plus neural gland of Ciona intestinalis, a
protochordate. Whereas GAD65 and GAD67 homologs were expressed in birds,
reptiles, and fish, only a single GAD cDNA with equal similarities to both
vertebrate GAD forms was found in the protochordate. This indicates that
the duplication of the vertebrate GAD gene occurred between 400 and 560
million years ago. For both GAD65 and GAD67, the generated phylogenetic
tree followed the general tree topology for the major vertebrate classes.
In turtle, an alternative spliced form of GAD65, putatively encoding a
truncated, nonactive GAD, was found. Furthermore, a third GAD form, which
is equally divergent from both GAD65 and GAD67, is expressed in C. (N.)
armatus. This third form might have originated from an ancient genome
duplication specific to modern ray-finned fishes.
ORIGINAL ARTICLE
Multiplicity of glutamic acid decarboxylases (GAD) in vertebrates: molecular phylogeny and evidence for a new GAD paralog
Department of Zoology, University of Aberdeen, U.K.
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