RNA transcripts were synthesized in vitro from a lambda DNA template with purified Escherichia coli RNA polymerase either in the presence or absence of the protein termination factor, rho. The products were initially characterized by electrophoresis on polyacrylamide slab gels, and two of the lower molecular weight discrete species (6 S and 4 S RNA) were further characterized by standard two-dimensional "fingerprint" analysis. Production of the 4 S RNA was strongly affected by the presence of

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... o, whereas production of the 6 S RNA species was relatively unaffected by rho. 3'-Terminal oligonucleotide fragments were then selectively isolated on columns of dihydroxyboryl-substituted cellulose from these transcripts. Sequence analysis of these oligonucleotide products indicated: (a) that all of the transcripts examined possess similar degrees of 3'-terminal sequence heterogeneity which consisted predominantly of the addition of 1 to 5 adenylate residues to the 3'-terminus of the transcript; and (b) that rho factor-enhanced termination results in a definite structural change in the nucleotide sequence with which an RNA molecule can terminate.