What contains most of tue mass in a atom

The nucleus contains the most mass.

RNA Polymerase II

Explanation:

In eukaryotes, the RNA polymerase responsible for the transcription of protein-encoding genes is the RNA polymerase II.

Generally in eukaryotes, there are three types of RNA polymerase. These include:

1. RNA polymerase I

2. RNA polymerase II

3. RNA polymerase II

The RNA polymerase I is responsible for transcribing ribosomal RNA genes except for 5S rRNA, RNA polymerase is responsible for transcribing protein-encoding genes into mRNA, while RNA polymerase II is responsible for transcribing transfer RNA and 5S ribosomal RNA genes.