On the relationship between preferred termination codon contexts and nonsense suppression in human cells

Area of Science:

• Genomics
• Molecular Biology
• Bioinformatics

Background:

• The 3' context of translation termination codons, specifically UAG, is crucial for gene expression regulation.
• Previous studies indicated context effects on nonsense suppression in human cells.
• Mammalian genomes exhibit regional variations in Guanine-Cytosine (G+C) content, known as isochores.

Purpose of the Study:

• To investigate if natural UAG stop codons in human genes show a preferred 3' sequence context.
• To determine if observed UAG contexts correlate with the G+C content of the surrounding genomic regions.
• To assess the influence of G+C composition on UAG context and its implications for translation.

Main Methods:

• Analysis of nucleotide sequences downstream of UAG codons in a diverse set of human genes.
• Classification of genes into five groups based on the G+C frequency at the third codon position.
• Comparison of base frequencies 3' to UAG codons across different G+C content groups.

Main Results:

• A significant variation in the frequency of bases immediately 3' to UAG codons was observed, correlating with the G+C content of the gene.
• Changes in 3' base composition were mirrored by alterations in the patterns of Guanine (G) and Adenine (A) containing dinucleotide (GN) and trinucleotide (AGN) strings.
• The observed patterns suggest genome-wide G+C composition dictates 3' base identity rather than selection for specific stop signal sequences.

Conclusions:

• The 3' context of human UAG stop codons is primarily influenced by regional genome-wide G+C composition.
• Selection for specific tetranucleotide stop signals does not appear to be the driving force behind UAG context.
• The lack of bias in human coding sequences may be an insufficient indicator for detecting codon usage or context effects during translation.