Figure 1.

Potential combinations of the three RNase H genes. The Venn diagram consists of three circles (HI, RNase HI; HII, RNase HII; HIII, RNase HIII) that represent the possible combinations of RNase H genes as Groups A to H. The 353 prokaryotic genomes were classified according to this system, and the results are presented in Additional File 1. The numbers under Groups A to H indicate the number of archaeal (left) and bacterial (right) species identified in each of the groups.