RNA chain elongation by RNA polymerase is a dynamic process. Techniques that 
allow the isolation of active elongation complexes have enabled investigators to 
describe individual steps in the polymerization of RNA chains. This article will 
describe recent studies of elongation by RNA polymerase II (pol II). At least 
four types of blockage to chain elongation can be overcome by elongation factor 
SII: (a) naturally occurring "arrest" sequences, (b) DNA-bound protein, (c) 
drugs bound in the DNA minor groove, and (d) chain-terminating substrates 
incorporated into the RNA chain. SII binds to RNA polymerase II and stimulates a 
ribonuclease activity that shortens nascent transcripts from their 3' ends. This 
RNA cleavage is required for chain elongation from some template positions. As a 
result, the pol II elongation complex can repeatedly shorten and reextend the 
nascent RNA chain in a process we refer to as cleavage-resynthesis. Hence, 
assembly of large RNAs does not necessarily proceed in a direct manner. The 
ability to shorten and reextend nascent RNAs means that a transcription 
impediment through which only half the enzyme molecules can proceed per 
encounter, can be overcome by 99% of the molecules after six iterations of 
cleavage-resynthesis. Surprisingly, the boundaries of the elongation complex do 
not move upstream after RNA cleavage. The physico-chemical alterations in the 
elongation complex that accompany RNA cleavage and permit renewed chain 
elongation are not yet understood.