title: Early assignments of the genetic code dependent upon protein structure.
abstract: Orgel (1972) has suggested that polynucleotides with sequences of alternating purine/pyrimidine are likely to have predominated in prebiotic conditions. Therefore, in any early template-directed protein synthesis, the number of available codons would have been limited. However, for any self-organizing system to survive and propagate, some feedback must occur from the products of the synthesis to the control of the synthetic procedure itself; i.e. the protein synthesized should have catalysed some step in the initation of template-directed synthesis. A given protein structure with a characteristic conformation and function would be optimal for the product of such a synthesis, and this in turn would limit the number of nucleotide sequences of those available able to give rise to a functioning synthetic assembly. A possible candidate for such an early polypeptide is the ferredoxin group of proteins and it is shown that with the present-day code the corresponding nucleotides do have a high percentage of alternating purine/pyrimidine sequences. Hence these combined restraints on the primitive synthetic machinery would direct the possible assignments of the genetic code helping to explain its regularity and universality.
mesh_terms: D000595:Amino Acid Sequence; D000596:Amino Acids; D001483:Base Sequence; D003062:Codon; D005815:Genetic Code; D014176:Protein Biosynthesis; D011487:Protein Conformation; D011506:Proteins; D011687:Purines; D011743:Pyrimidines; D014158:Transcription, Genetic
chemical_list: D000596:Amino Acids; D003062:Codon; D011506:Proteins; D011687:Purines; D011743:Pyrimidines