Cells or nuclei are incubated with BrUTP in the presence of sarkosyl, which prevents the attachment of RNA polymerase to the DNA. Therefore only RNA polymerase that are already on the DNA before the addition of sarkosyl will produce new transcripts that will be labeled with BrU.
Cells or nuclei are exposed to BrUTP along with sarkosyl, which inhibits the attachment of RNA polymerase to DNA. This leads to the production of new transcripts by RNA polymerase that were already bound to DNA before the addition of sarkosyl.