Sequence and structural evolution of the KsgA/Dim1 methyltransferase family

One of the 60 or so genes conserved in all domains of life is the ksgA/dim1 orthologous group. Enzymes from this family perform the same post-transcriptional nucleotide modification in ribosome biogenesis, irrespective of organism. Despite this common function, divergence has enabled some family members to adopt new and sometimes radically different functions. For example, in S. cerevisiae Dim1 performs two distinct functions in ribosome biogenesis, while human mtTFB is not only an rRNA methyltransferase in the mitochondria but also a mitochondrial transcription factor. Thus, these proteins offer an unprecedented opportunity to study evolutionary aspects of structure/function relationships, especially with respect to our recently published work on the binding mode of a KsgA family member to its 30S subunit substrate. Here we compare and contrast KsgA orthologs from bacteria, eukaryotes, and mitochondria as well as the paralogous ErmC enzyme.

By using structure and sequence comparisons in concert with a unified ribosome binding model, we have identified regions of the orthologs that are likely related to gains of function beyond the common methyltransferase function. There are core regions common to the entire enzyme class that are associated with ribosome binding, an event required in rRNA methylation activity, and regions that are conserved in subgroups that are presumably related to non-methyltransferase functions.

The ancient protein KsgA/Dim1 has adapted to cellular roles beyond that of merely an rRNA methyltransferase. These results provide a structural foundation for analysis of multiple aspects of ribosome biogenesis and mitochondrial transcription.