Bacteria use attached to the polymerase and they direct rna polymerase to promoters; eukaryotic cells use that instead attach directly to the dna first.

Bacteria use sigma factors attached to the polymerase and they direct RNA polymerase to promoters. Eukaryotic cells use transcription factors that instead attach directly to the DNA first.

Sigma factors is a specific protein that bacteria uses to initiate the binding of RNA polymerase by directing it to the DNA. Eukaryotes use transcription factors which initiate the binding of these proteins to the DNA to signal transcription. 

They both determine the exact age of rocks.