The β2 subunits of tryptophan synthetase, formula α2β2, from Escherichia coli, Shigella dysenteriae, Enterobacter aerogenes, Salmonella typhimurium, and Serratia marcescens were compared by three criteria. (i) αβ association constants for the various β2 subunits and E. coli α subunit varied between 3.6 × 108m−1 for E. coli and 0.33 × 108m−1 for S. marcescens; values for the other organisms were intermediate. (ii) Antiserum neutralization of the β2 subunit enzyme activity using anti-E. coli β2 serum showed significant cross-reaction among the organisms (E. coli, 1.0; S. dysenteriae, 0.98; S. typhimurium, 0.67; E. aerogenes, 0.61; S. marcescens, 0.42). (iii) Quantitative microcomplement fixation showed E. coli β2 and S. marcescens β2 subunits to have an index of dissimilarity of 1.8 while the other organisms had intermediate indexes. Similar complement fixation data were obtained with antisera from separate rabbits and from first course and boost sera. These findings suggest that the general surface structure and the respective α subunit binding site of the β2 subunits from these Enterobacteriaceae have been strongly conserved
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