Initiation and elongation of RNA synthesis in prokaryotes
RNA synthesis by RNA polymerase proceeds in four steps : (i) The holoenzyme first binds at the promoter site, forming the closed promoter complex in which DNA remains double helical, (ii) The closed complex isomerizes, and causes unwinding and separation of DNA strands to form open (binary) promoter complex, (iii) After unwinding, only one of the two strands is copied; this is achieved by incorporation of nucleotides, initially without movement of enzyme leading to the formation of RNA chain, upto 9 bases in length. During the incorporation of these 9 bases, at every step, there is a possibility for the release of. this small RNA chain, a process described as 'abortive initiation'. A cycle of abortive initiations usually 'occurs generating a series of short (2-9 base) oligonucleotides, before initiation is actually Successful. (iv) Once initiation succeeds, the sigma factor of RNA polymerase dissociates. (v) The dissociation of σfactor marks the entry of NusA protein, which helps elongation, and promotes pausing and termination at specific sites. Core enzyme now undergoes a major conforraational rearrangement and a stable ternary elongation complex is formed. This complex moves along DNA, synthesizing RNA all along its path at a rate of about 40 bases per second at 37°C. RNA polymerases in some phages (T3, T7) are each a polypeptide (< 100,000 daltons), which synthesizes RNA very rapidly (~200 bases/second at 37°C). Elongation of RNA transcript continues till an unstable termination complex is formed (see later for termination).