Why does RNA synthesis in E.coli begin with a Purine? From Berg's Biochemistry: > Most newly synthesized RNA chains carry a highly distinctive tag on the 5′ end: the first base at that end is either pppG or pppA. Is there a reason why the synthesis initiates with a purine?

> Purine-rich sequences at the 5' ends of the nascent transcripts of many genes act to stabilize the early ternary(elongation) complex.

P.S.Source: MattDMo's comment.