RNA polymerases (RNAPs) carry out transcription in the three domains of life, Bacteria, Archaea, and Eukarya. Transcription initiation is highly regulated by a variety of transcription factors, whose number and subunit complexity increase during evolution. This process is regulated in Bacteria by the sigma factor, while the three eukaryotic RNAPs require a complex set of transcription factors (TFs) and a TATA-binding protein (TBP). The archaeal transcription system appears to be an ancestral version of the eukaryotic RNAPII, requiring transcription factor B (TFB), TBP, and transcription factor E (TFE). The function of the bacterial sigma (sigma) factor has been correlated to the roles played by the eukaryotic RNAP II and the archaeal RNAP. In addition, sigma factors, TFB, and TFIIB all contain multiple DNA binding helix-turn-helix (HTH) structural motifs; although TFIIB and TFB display two HTH domains, while the bacterial sigma factor spans 4 HTH motifs. The sequence similarities and structure alignments of the bacterial sigma factor, eukaryotic TFIIB, and archaeal TFB evidence that these three proteins are homologs.Key Points center dot Transcription initiation is highly regulated by TFs.center dot Transcription is finely regulated in all domains of life by different sets of TFs.center dot Specific TFs in Bacteria, Eukarya and Archaea are homologs.