Is RNA polymerase a holoenzyme?

Yes, RNA polymerase is a holoenzyme in all organisms except for E. coli.

A holoenzyme is a large preassembled complex containing an enzyme and its coenzyme. RNA polymerase consists of a core enzyme made up of 5 polypeptide subunits. The interaction between this core enzyme and a sigma factor results in an RNA polymerase holoenzyme. The presence of the sigma factor enables RNA polymerase holoenzyme to detect specific promoter sequences and activate transcription in a wide range of conditions. Transcription starts when the RNA polymerase holoenzyme locates and binds to promoter DNA.

RNA polymerase in E. coli consists of only the core enzyme. It is made up of the 5 polypeptide subunits but is lacking the sigma factor and is therefore not a holoenzyme.