This single subunit RNA polymerase is the smallest of the well-characterized RNA polymerases and appears evolutionarily unrelated to the multi-subunit family of RNA polymerases from bacteria and eukaryotes. It is instead related to the Pol I family of polymerases (bacterial DNA Pol I and various RNA-dependent RNA polymerases and reverse transcriptases). Curiously, it is also related to the eukaryotic mitochondrial and chloroplast RNA polymerases.
Examination of the structure suggests two independently folded domains:
Let's look at the between the N- and C-terminal domains.
The C-terminal domain is homologous to the Pol I family of polymerases. The N-terminal domain appears to be a "swappable" unit - promoter recognition in the RNA polymerase, but proofreading in DNA Pol I.