Transcription is the first stage of gene expression, accomplished in
all organisms by multi-subunit RNA polymerase. RNA polymerase is
large, complex and evolutionary conserved molecular machine. The
active centre of RNA polymerase is able to perform two opposite
reactions – synthesis of the transcript and its hydrolysis. Hydrolysis
of the RNA is the main mechanism of transcriptional proofreading. RNA
polymerase is not very efficient in this reaction and requires
assistance of special proofreading factors.  In recent years it became
clear that the role of the proofreading factors goes far beyond
production of the correct RNA. In the cellular context they prevent
transcriptional “traffic jams” and collision of transcribing RNA
polymerase with replication fork. The vast majority of bacteria encode
proofreading factors, including free-living bacteria with smallest
genomes. The notable exclusion is cyanobacteria. Currently we are
trying to understand mechanisms, which cyanobacteria use to compensate
for the absence of proofreading factors.