Gene arrangements and branching orders of gram-positive bacteria

The availability of complete genomic sequence data allows one to develop new methods of reconstructing phylogenetic trees. A simple method of reconstructing branching orders based on gene transposition (or lateral transfer) is presented. It is argued that specific gene arrangements on four different genomes could determine a branching order. A computer search for such gene arrangements was carried out against gene order data of completely sequenced Gram-positive bacteria. Gene arrangements around ribosomal protein S4 gene, murC (UDP-N-acetylmuramate:alanine ligase) gene and dnaE (DNA polymerase III α chain) gene each suggest a branching order in which actinobacteria with a high genomic G+C content first branched off from other Gram-positives with a low G+C content and then a split occurred between Mycoplasma species and a group closely related to Bacillus subtilis. A recently sequenced thermophilic bacterium Thermoanaerobacter tengcongensis is suggested to have branched off from the lineage leading to the low G+C Gram-positives prior to the split between the Mycoplasma and Bacillus groups. By contrast to the indel analysis in which a single evolutionary event of insertion or deletion of a signature sequence is assumed, the present method does not necessarily require such a parsimonious assumption of gene transposition.