Figure 1.

Pseudoknot structure of the S15 messenger site and regulatory properties of mutations in stem 1. Base pair changes were introduced in a rpsO–lacZ translational fusion carried in a λ phage. For each mutation, the effect of overexpression of S15 on the level of β-galactosidase was measured and compared with the value of a strain carrying a wild-type fusion (10). The mutated base pairs indicated are described in Table 1. Repression ratios correspond to the ratio of the β-galactosidase units in the control divided by the value in the mutant and are given in the squares. This ratio is 1 when all control is lost and is 22 for the wild type (WT). The recognition determinant, G⋅U/G⋅C, is boxed, and the adjacent base pairs are ovalized. Ribose protection of the messenger binding site against ethylnitrosourea (ENU) in the presence of S15 are taken from ref. 13 and are shown by circles: •, strong protection; ○, weak protection. +1 corresponds to the translational initiation codon.