B2 RNA and 7SK RNA, RNA polymerase III transcripts, have a cap-like structure at their 5′ end

We found that hydrolysates of poly(A) + ; RNA from Ehrlich ascites carcinoma cells which were transcribed by RNA polymerase III contained an unusual component designated as X. It was part of B2 RNA representing a transcript of B2 retroposon, typical of rodents. The component X possesses a cap-like structure, xppp5′G, where x has a non-nucleotide structure. About half of all B2 RNAs contained this group at the 5′ end. Previously, Epstein et al. (1) detected a similar structure at the 5′ end of small nuclear U6 RNA. Later, Singh and Reddy (2) showed methyl to be the blocking group in the component x of U6 RNA. Besides B2 RNA, we found 5′ ends containing methyl groups in 7SK RNA.