Divergent Evolution of 5 S rRNA Genes in Methanococcus

Abstract

Abstract The organization of genes for 5S rRNA in the methanogenic archaebacterium Methanococcus (M .) voltae and their nucleotide sequences have been determined. M. voltae possesses three 5S rRNA genes, one of them is organized in an rRNA transcriptional unit coding for 16S-23S-5S rRNA. The other two are associated with seven tRNA genes in a putative transcriptional unit composed of 5′-tRNA Thr -tRNA Pro -tRNA Tyr -tRNA Lys - 5S rRNA-tRNA Asp -tRNA Lys - 5S rRNA-tRNA Asp -3′. Coding regions plus spacers of the tRNA Lys -5S rRNA-tRNA Asp block of this gene cluster occur twice with identical sequence. The 5S rRNA from this cluster displays considerable sequence divergence to the rRNA operon-linked 5S rRNA gene. Comparison of the M. voltae 5S rRNA sequences with those from M. vannielii revealed that the operon-linked genes on one hand and the tRNA-linked 5S genes on the other share a greater sequence homology than the two types of genes within each of the two organisms. This indicates an independent evolution of the two sets of 5S rRNA genes without selective pressure from other ribosomal components or, alternatively, lateral gene transfer.