How does RNA polymerase achieve higher processivity?

DNA polymerase uses a sliding clamp in order to replicate DNA. RNA polymerase does not require one. What allows RNA polymerase to hold on to DNA just like DNA polymerase?

The structure in the presence of s (holoenzyme) is on the right; note the open channel for DNA binding. The structure in the absence of s (the core enzyme) is on the left.  Note that the channel is now closed, as if the fingers and thumbs of a hand now closed to make a circle. This striking conformational change that occurs when s dissociates is thought to confer high processivity on the RNA polymerase.
SOURCE: http://biowiki.ucdavis.edu/Genetics/Unit_III%3A_The_Pathway_of_Gene_Expression/10%3A_Transcription%3A_RNA_polymerases