Origin and Mechanism of Evolution of Antifreeze Glycoproteins in Polar Fishes
323
Nonhomologous Gene Structure
Homologous genes, i.e. genes that share a common ancestor, are highly
conserved in terms of the number of exons and introns and their positions.
This is clearly not the case between the AFGP genes of the two fishes.
Arctic cod AFGP genes have three exons and two introns, and the introns
intervene in the 5'UTR and between translation start and signal peptide
(Fig. 4A). By contrast, notothenioid AFGP genes have two exons and a
single large intron, and the intron intervenes between signal peptide and
the AFGP polyprotein (Fig. IA). The AFGP genes of the two fishes are
thus not homologous.
Distinct AFGP Tripeptide Coding Sequences
The codons for the tripeptide repeats in Arctic cod AFGP genes have a
very different bias from that of the notothenioids. The sequence
composition of one of the Bs genes is as follows:
Thr - Ala
48%act
54%gca
47%aca 33%gcg
Ala
54%gca
38%gcc
Each of the three amino acids is encoded primarily by two codons, one
of which (highlighted) is seldom or never used in notothenioid AFGP
genes (Fig. 2). The Pro that replace the first Ala are encoded by either cca
(50%) or ccg (48%) and very likely resulted from a I-nt substitution of gca
or gcg. By contrast, the Pro in this position in notothenioid AFGPs are
encoded exclusively by cct, a result of a I-nt substitution from gct (not
used in Arctic cod) to cct (Fig. 2). These differences indicate that the
Arctic cod AFGP repeats are derived from duplications of an unrelated
ancestral ThrAlaAla coding element.
Unrelated Spacer Sequence
The spacers of the two groups of genes share no sequence similarity at
both the nucleotide and protein level and are clearly unrelated. The distinct
spacers in fact invoke the use of a different protease in the processing of
the polyprotein precursor. While the origin of the notothenioid spacers is
unknown, the spacers in Arctic cod clearly arose from a I-nt transversion,
aca to aga, converting a Thr to an Arg, since the Arg residues are
exclusively encoded by aga, and always replace a Thr in the repeats [30].
323
Nonhomologous Gene Structure
Homologous genes, i.e. genes that share a common ancestor, are highly
conserved in terms of the number of exons and introns and their positions.
This is clearly not the case between the AFGP genes of the two fishes.
Arctic cod AFGP genes have three exons and two introns, and the introns
intervene in the 5'UTR and between translation start and signal peptide
(Fig. 4A). By contrast, notothenioid AFGP genes have two exons and a
single large intron, and the intron intervenes between signal peptide and
the AFGP polyprotein (Fig. IA). The AFGP genes of the two fishes are
thus not homologous.
Distinct AFGP Tripeptide Coding Sequences
The codons for the tripeptide repeats in Arctic cod AFGP genes have a
very different bias from that of the notothenioids. The sequence
composition of one of the Bs genes is as follows:
Thr - Ala
48%act
54%gca
47%aca 33%gcg
Ala
54%gca
38%gcc
Each of the three amino acids is encoded primarily by two codons, one
of which (highlighted) is seldom or never used in notothenioid AFGP
genes (Fig. 2). The Pro that replace the first Ala are encoded by either cca
(50%) or ccg (48%) and very likely resulted from a I-nt substitution of gca
or gcg. By contrast, the Pro in this position in notothenioid AFGPs are
encoded exclusively by cct, a result of a I-nt substitution from gct (not
used in Arctic cod) to cct (Fig. 2). These differences indicate that the
Arctic cod AFGP repeats are derived from duplications of an unrelated
ancestral ThrAlaAla coding element.
Unrelated Spacer Sequence
The spacers of the two groups of genes share no sequence similarity at
both the nucleotide and protein level and are clearly unrelated. The distinct
spacers in fact invoke the use of a different protease in the processing of
the polyprotein precursor. While the origin of the notothenioid spacers is
unknown, the spacers in Arctic cod clearly arose from a I-nt transversion,
aca to aga, converting a Thr to an Arg, since the Arg residues are
exclusively encoded by aga, and always replace a Thr in the repeats [30].