Covariance of complementary rRNA loop nucleotides does not necessarily represent functional pseudoknot formation in vivo

Mutational analysis of the ribosome

The ribosome is responsible for protein synthesis, the translation of the genetic code, in all living organisms. Ribosomes are composed of RNA (ribosomal RNA) and protein (ribosomal protein). Soluble protein factors bind to the ribosome and facilitate different phases of translation. Genetic approaches have proved useful for the identification and characterization of the structural and functional roles of specific nucleotides in ribosomal RNA and of specific amino acids in ribosomal proteins and in ribosomal factors. This chapter summarizes examples of mutations identified in ribosomal RNA, ribosomal proteins, and ribosomal factors.

A tertiary structural element in S box leader RNAs is required for S-adenosylmethionine-directed transcription termination

The S box transcription termination control system regulates expression of genes involved in methionine metabolism. Expression of the S box regulon, comprised of 11 transcriptional units in Bacillus subtilis, is induced in response to starvation for methionine. We previously demonstrated that S-adenosylmethionine (SAM) is the molecular effector sensed by the S box leader RNAs during transcription. A secondary structure model for S box leader RNAs was developed based on conservation of primary sequence elements and sequence covariation in helical domains. Covariation of nucleotides in two distantly spaced unpaired regions in the S box leader RNAs suggested that these two domains might interact in the RNA tertiary structure. In this study, site-directed mutagenesis of the covarying residues in two B. subtilis S box leader sequences was employed to test the hypothesis that base-pairing between these regions may be important. The effect of these mutations on in vivo expression, transcription termination in vitro, SAM binding, and leader RNA structure strongly supported the model that interaction between these two regions plays a key role in S box leader function. This provides the first insight into the three-dimensional arrangement of structural elements within the S box RNAs.