Clues to evolution of the genetic code can be found by comparing usage of 
anticodons in various organisms and organelles. GC content of DNA varies, as a 
result of directional mutation pressure (AT/GC pressure), especially in 
bacteria. Low GC in Mycoplasma is accompanied by use of UGA for tryptophan and, 
in ciliated protozoa, by use of UAA and UAG for glutamine. These are examples of 
"stop codon capture," which has been preceded by duplication of tRNA genes 
followed by nucleotide substitutions in their sequences, including mutational 
changes in their anticodons. Evolutionary changes in the code may have resulted 
from disappearance of codons and anticodons resulting from GC pressure and from 
their reappearance when the direction of the pressure was reversed. In this 
manner, codon UGA and anticodon UCA for tryptophan could have disappeared under 
GC pressure and reappeared in Mycoplasma under AT pressure. Stop codon UGA may 
have been the third of the three stop codons to appear, originating from 
mutations in UAA. Changes in the code are adaptive and nondeleterious. We 
propose that the number of anticodons has increased and that evolution continued 
until three existing forms of the universal code were produced: eukaryotic, 
eubacterial, and the code for halobacteria and methanococci. These three codes 
are distinguished from each other by their anticodon pattern. The eukaryotic 
code contains eight INN (ANN) anticodons that have replaced GNN anticodons as a 
result of AT pressure. Mitochondrial and chloroplast codes have evolved from the 
eubacterial code through genomic economization and AT pressure, leading to 
losses of GNN and CNN anticodons.(ABSTRACT TRUNCATED AT 250 WORDS)