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Gene Expression Help (page 2)

By — McGraw-Hill Professional
Updated on Aug 23, 2011

Translation

The process of translating an mRNA into a polypeptide chain in eukaryotes is essentially the same as that in bacteria, but differs in several important ways. Whereas only three well-defined initiation factors are required for translation of E. coli mRNAs, many more are needed in eukaryotes. Eukaryotic initiation factors are designated eIFs to distinguish them from their bacterial counterparts.

EXAMPLE 13.2 A tRNAMet (symbolized Met-tRNAMet when activated) brings an unformylated methionine into the first position on the ribosome. Hydrolysis of ATP to ADP is required for mRNA binding. The 40S ribosomal subunit is then thought to attach to the mRNA at its capped 5' terminus, and then it slides along (consuming ATP) until it reaches the first AUG codon. Normally, only AUG is an efficient initiator codon in eukaryotes, whereas UUG, GUG, and AUU may also be used in E. coli.

EXAMPLE 13.3 Three different elongation factors (EFs) in eukaryotes replace those found in bacteria. However, a single termination factor (RF) replaces RF1 and RF2 of bacteria. RF recognizes all three stop codons (UAC, UAA, and UGA).

Posttranslation Modifications

A nascent polypeptide chain may not become biologically active until after it has been modified in one or more specific ways, such as being enzymatically phosphorylated, glycosylated, or partly digested by peptidase enzymes. Phosphorylation involves the addition of one or more phosphate groups (Example 13.4) and glycosylation involves the addition of one or more carbohydrate groups to the protein sequence. Peptidase enzymes cleave the protein into smaller units (Example 13.5).

EXAMPLE 13.4 Protein kinases are enzymes that transfer terminal phosphate groups from ATP to specific amino acids on target proteins. Phosphorylation of these proteins may either raise or lower their biological activities. For example, the skeletal muscle enzyme glycogen synthetase is inactivated after phosphorylation, whereas phosphorylation of the enzyme glycogen phosphorylase increases its activity.

EXAMPLE 13.5 The hormone insulin is synthesized as a single-chain precursor (proinsulin) with little or no hormonal activity. Two internal cuts remove 31 amino acids from pro-insulin, producing the two polypeptide chains of the functional dimer that are held together by disulfide bonds. Likewise, human growth hormone that circulates in blood is a ''clipped'' version of the pituitary form of that hormone.

Practice problems for these concepts can be found at:

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