Antisense RNA-mediated transcriptional attenuation in plasmid pIP501: the simultaneous interaction between two complementary loop pairs is required for efficient inhibition by the antisense RNA.

Streptococcal plasmid pIP501 uses antisense RNA-mediated transcriptional attenuation to regulate its replication. Previous in vitro assays suggested that binding intermediates between RNAII (sense RNA) and RNAIII (antisense RNA) are sufficient for inhibition, and a U-turn structure on RNAII loop L1 was found to be crucial for the interaction with RNAIII. Here, sequence and structural requirements for an efficient RNAII-RNAIII interaction were investigated. A detailed probing of RNA secondary structure combined with in vitro single-round transcription assays indicated that complex formation between the two molecules progresses into the lower stems of both loop pairs of the sense and antisense RNAs, but that the complex between RNAII and RNAIII is not a full duplex. Stem-loops L3 and L4 were required to be linked to one other for efficient contact with the complementary loops L2 and L1 of the sense RNA, indicating a simultaneous interaction between these two loop pairs. Thereby, the sequence and length of the spacer connecting L3 and L4 were shown not to be important for inhibition.