Plant Biotechnology and Genetics: Principles, Techniques and Applications

Since an mRNA is a long molecule containing many nucleotides, where does the trans-
lational machinery start looking to begin reading each codon? The first codon read is called
theinitiation(orstart)codon, and it is usually AUG that codes for methionine [earlier we
mentioned that the protein-coding portion of the gene (DNA) began with ATG]. In eukar-
yotes, the initiation codon is surrounded by a consensus sequence termed theKozak
sequence(ACCAUGG) (Kozak 1986, 1987), which indicates to the translational machinery
to begin translation with this codon. If this sequence is not present, this codon will be
missed and the cellular machinery will continue to scan down the mRNA until it finds a
suitable initiation codon, if present. As mentioned above, three different reading frames
are possible. The start codon defines what the correct reading frame will be for any particu-
lar gene. As you will see later, this is an important consideration for biotechnology.

6.4.1 Initiation of Translation

Translation of the mRNA is done in connection with organelles calledribosomesand
another type of RNA termed transfer RNA (tRNA). In eukaryotes, ribosomes are
complex and composed of two subunits, one large and the other small. The large
subunit contains three types of ribosomal RNAs (rRNAs) (28SrRNA, 5SrRNA, and
5.8SrRNA), along with 49 proteins. The small subunit contains the 18SrRNA, and 33 pro-
teins. A ribosome will bind to the 5^0 end of the mRNA and move down toward the 3^0 end as
translation proceeds. Specifically, starting at the 5^0 cap of the mRNA, the small subunit of
the ribosome along with initiation factors will bind and move down the mRNA until it
encounters the proper initiation codon. Then the correct amino acid (the initiation codon
codes for methionine; therefore methionine is always the first amino acid in the initial poly-
peptide) is brought to it via a tRNA molecule and combines with additional factors to form
an initiation complex. The tRNA molecule is said to be “charged” when it carries an amino

TABLE 6.2. The Genetic Code—mRNA Codons and Amino Acids Encodeda

First Base

Second Base

UCA GSecond Base

U

UUU Phe UCU

Ser

UAU Tyr UGU Cys U

UUC UCC UAC UGC C

UUA Leu UCA UAA Stop UGA Stop A

UUG UCG UAG UGG Trp G

C

CUU

Leu

CCU

Pro

CAU His CGU

Arg

U

CUC CCC CAC CGC C

CUA CCA CAA Gln CGA A

CUG CCG CAG CGG G

A

AUU

Ile

ACU

Thr

AAU Asn AGU Ser U

AUC ACC AAC AGC C

AUA ACA AAA Lys AGA Arg A

AUG Met ACG AAG AGG G

G

GUU

Val

GCU

Ala

GAU Asp GGU

Gly

U

GUC GCC GAC GGC C

GUA GCA GAA Glu GGA A

GUG GCG GAG GGG G

aThe codons are written in the 5 (^0)! 30 direction.
150 MOLECULAR GENETICS OF GENE EXPRESSION