Relationship between the occurrence of cysteine in proteins and the complexity of organisms

The occurrence and relative positions of cysteine residues were investigate d in proteins of various species. Considering random mathematical occurrenc e for an amino acid coded by two codons (3.28%), cysteine is underrepresent ed in all organisms investigated. Representation of cysteine appears to cor relate positively with the complexity of the organism, ranging between 2.26 % in mammals and 0.5% in some members of the Archeabacteria order. This obs ervation, together with the results obtained from comparison of cysteine co ntent of various ribosomal proteins, indicates that evolution takes advanta ge of increased use of cysteine residues. In all organisms studied except p lants, two cysteines are frequently found two amino acid residues apart (C- (X)(2)-C motif). Such a motif is known to be present in a variety of metal- binding proteins and oxidoreductases. Remarkably, more than 21% of all of c ysteines were found within the C-(X)(2)-C motifs in Archea. This observatio n may indicate that cysteine appeared in ancient metal-binding proteins fir st and was introduced into other proteins later.