We report the results of a novel method for locating sites of transcription initiation using a complex of capping enzymes from vaccinia virions that catalyze the reaction pppG + S -adenosylmethionine + (p)ppXpYpZp..... → 7m GpppXpYpZp..... [Ensinger, M. J., Martin, S. A., Paoletti, E. and Moss, B. (1975) Proc. Natl. Acad. Sci. USA 72, 2525-2529]. This enzyme complex will cap di- or triphosphate termini but will not cap monophosphate or hydroxyl termini. Xenopus laevis 40S precursor rRNA from oocytes is capped by these enzymes, and we conclude that it has 5′-polyphosphate termini. Therefore, 40S RNA must represent the primary transcript of amplified X. laevis ribosomal DNA. The majority of 40S molecules with polyphosphate termini begin with the sequence (p)ppAAG. There is evidence, however, that the 5′ terminus may be heterogeneous. The majority of all detectable initiation events were localized close to the region coding for the 5′ end of the 40S RNA. No initiation sites were detected in the nontranscribed spacer, but an apparent initiation site in the middle of the transcribed region was also observed.

Reeder, R. H., Sollner-Webb, B., & Wahn, H. L. (1977). Sites of transcription initiation in vivo on Xenopus laevis ribosomal DNA. Proceedings of the National Academy of Sciences, 74(12), 5402–5406.