The prokaryotic genetic code has been influenced by directional mutation 
pressure (GC/AT pressure) that has been exerted on the entire genome. This 
pressure affects the synonymous codon choice, the amino acid composition of 
proteins and tRNA anticodons. Unassigned codons would have been produced in 
bacteria with extremely high GC or AT genomes by deleting certain codons and the 
corresponding tRNAs. A high AT pressure together with genomic economization led 
to a change in assignment of the UGA codon, from stop to tryptophan, in 
Mycoplasma.