Messenger RNA conformation and ribosome selection of translational reinitiation sites in the lac repressor mRNA.

In the early region of the Escherichia coli lac repressor mRNA, the pattern of cleavage by nucleases specific for single or double-stranded RNA confirms the presence of secondary structures previously proposed to influence the pattern of translational reinitiation. These are positioned so as to mask a potential restart site centered on an in-phase GUG triplet corresponding to repressor amino acid position Val38. Our finding that a restart polypeptide initiated at the Val38 GUG codon is observed only in situations that that preclude base-pairing of adjacent mRNA sequences establishes a functional role for these structures in vivo. This evidence for structure, considered with the overall pattern of reinitiation events, suggests that local mRNA conformation is the major determinant that dictates ribosomal selection of restart sites within the early region of the repressor cistron.