Spectinomycin operon ofMicrococcus luteus: Evolutionary implications of organization and novel codon usage

Summary The complete DNA sequence of theMicrococcus luteus spectinomycin (spc) operon and its adjacent regions has been determined. The sequence has revealed the presence of genes that are homologous to those of theEscherichia coli ribosomal and related proteins, L14, L24, L5, S8, L6, L18, S5, L30, L15, and secretion protein Y (secY), and the gene for adenylate kinase (adk). The gene arrangement in the spc operon is essentially the same as that ofE. coli except for the absence in theM. luteus spc operon of the genes for S14 and X protein that exist in theE. coli spc operon.SecY andadk seem to be composed of another operon (adk operon) with at least an open reading frame. The deduced amino acid sequences for these ribosomal proteins are well conserved among the two species (40–65% identity). Reflecting the high genomic guanine and cytosine (GC) content ofM. luteus (74%), the codon usage of the genes is extremely biased toward use of G and C, about 94% of the codon third positions being G or C. Seven codons, AUA, AAA, AGA, UUA, GUA, CUA, and CAA, all of which have A at the codon third positions, are completely absent in theM. luteus genes examined. Out of 11 genes in theM. luteus spc and adk operons, 5 (10) use GUG (UGA) and 6 (1) use AUG (UAA) as an initiation (termination) codon.