This prediction is supported by mutational studies and the pattern of sensitivity of the two RNAs to cleavage with single- and double-strand-specific endoribonucleases. RNA structure is critical to antitermination because mutations that prevent the formation of base pairs in the stems reduce function, and these mutations can be suppressed by additional mutations that restore base pairing. Like lambda N and Q, the PUT sequences suppress polymerase pausing and promote processive antitermination in a purified in vitro transcription system.