Sigma factors and transcription factors are gene assignments that are use across all organisms. Gene regulation is everywhere! They are not unique to prokaryotes, and as such, they have diverse means in the context of, at the very least, prokaryotes vs eukaryotes. Today, May 30, 2020, we are going to avoid them all together and call such hits "DNA binding proteins" for now. One of the exceptions is WhiB. We will still call "WhiB family transcription factor" when appropriate.