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.