Genes and gene families - Human Gene Evolution - page 176

PABP

P-rich domain

Splicing
factors

hnRNPs

RNA-binding
domain

G-rich domain

snRNP U1-70K

R-rich domain
Basic acidic domain

Nucleolin

Acidic domain
G-rich domain

Figure 4.23. Evolutionary schema for various types of RNA-binding protein (redrawn
from Fukami-Kobayashi et al ., 1993). PABP: poly(A) binding protein. hnRNP:
heterogeneous nuclear ribonucleoprotein. snRNP: small nuclear ribonucleoprotein.

Genes encoding tRNAs and aminoacyl-tRNA synthetases. tRNA mole-
cules perform a central role in protein biosynthesis by potentiating the transfer
of amino acids to the ribosome. Each amino acid is bound to the tRNA by an
aminoacyl-tRNA synthetase. Each tRNA contains sequence elements (located
in the anticodon, acceptor stem or the discriminator base at position 73) that
are unambiguously recognized by its cognate synthetase. The tRNA-tRNA
synthetase complex, which also includes an elongation factor and GTP, enters
the ribosome where the anticodon of the tRNA interacts specifically with one
of the codon triplets of the mRNA. Subsequently, the amino acid is covalently
bound to the C-terminal of the nascent polypeptide chain. tRNAs have similar
sizes and tertiary structures (Mans et al ., 1991) but are subdivided into isoac-
cepting groups which, despite having different sequences, recognize the same
amino acid.
It is unclear how many functional tRNA genes are present in the human
genome. An early saturation hybridization study put the total number of tRNA
genes at about 1300 per haploid human genome, an average of 65 copies for each
tRNA species (Hatlen and Attardi, 1971). Chromosomally allocated human
tRNA genes include TRAN (alanine, 6p21-p22), TRE (glutamic acid, 1p36),
TRG1 (glycine, chromosome 1), TRK1 (lysine, 17p13), TRL1 (leucine 1, 14q11-
q12), TRL2 (leucine 2, 17p13), TRMI1 and TRMI2 (methionine, 6p23-q12),
TRN (asparagine, 1p36), TRP1 and TRP2 (proline 1 and 2, 14q11-q12), TRP3
(proline 3, chromosome 5), TRQ1 (glutamine 1, 17p13), TRR1 (arginine 1,
17p13), TRR3 (arginine 3, 6p21-p22), TRR4 (arginine 4, 6p21-p22), TRT1 (thre-
onine 1, chromosome 5), and TRT2 (threonine 2, 14q11-q12). Thus tRNAs from
different isoaccepting groups often cluster together, for example 6p22 ( TRAN ,
TRMI1 , TRMI2 , TRR3 , TRR4 ; Buckland et al ., 1996), 14q11-q12 ( TRL1 , TRP1 ,
TRP2 , TRT2 ; Chang et al ., 1986) and 17p13 ( TRK1 , TRL2 , TRQ1 , TRR1 ;
Morrison et al ., 1991). Other tRNA genes are dispersed and, since these genes are