This work was supported in part by the CNRS interdisciplinary program Microbiologie Fondamentale. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. □ S The on-line version of this article (available at http://www.jbc.org) contains supplemental Fig. S1 . The atomic coordinates and structure factors (code 2HX6) have been

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... eposited in the Protein Downloaded from 5 The abbreviations used are: TOCSY, total correlation spectroscopy; NOE, nuclear Overhauser effect; NOESY, nuclear Overhauser effect spectroscopy; HSQC, heteronuclear single quantum coherence. Downloaded from FIGURE 4. Comparison of T4 RegB sequence with those of other phages and of non-toxic mutants. We compare the most distant sequences, namely those of T4, Tu1a, 44RR, and RB49 (conserved amino acids are indicated in red). We also report the sequences of the mutants (T4-M1 to T4-M11) obtained in the random-mutagenesis experiment (the mutated positions are in yellow). Finally we indicate the location of the point mutants constructed to assess the role of the residues we thought involved in RNA binding and/or processing (point). The E21A mutant, which has no influence on RegB toxicity, is indicated in green. The secondary structure elements are indicated above the sequences (␣-helices in red, and ␤-strands in blue).