FIG 6.

BPA-mediated cross-linking of Crl to σ^S. (A and B) SDS-PAGE gels showing the results of UV exposure on ³²P-σ^S(F79BPA) (A) or ³²P-σ^S(WT) (B) incubated with either WT Crl or Crl variants with the photoreactive amino acid analog BPA incorporated at the indicated positions. Free ³²P-σ^S and cross-linked ³²P-σ^S-Crl complexes are indicated with arrows. Crl-BPA variants did not produce higher-molecular-weight bands in the absence of UV treatment or without σ^S. (C and D) Surface representations of the E. coli Crl homology model. Panel D is a 180° rotation of panel C. The positions of BPA substitutions in panels A and B are labeled and colored: orange, Crl-BPA variants that cross-link strongly to σ^S(WT); dark gray, Crl-BPA variants that cross-link very weakly or not at all to σ^S(WT); teal, residues in the central cleft patch proposed to interact with σ^S (Fig. 3B shows the identities of residues in addition to R51).