The Rate of Evolutionary Divergence of Initiation Factors IF2 and IF3 in Various Bacterial Species Determined Quantitatively by Immunoblotting

Antibodies to Escherichia coli translational initiation factors IF2 and IF3 were used for an immunological comparison of unpurified proteins from the following genera: Salmonella, Serratia, Proteus, Aeromonas, Pseudomonas, Streptococcus, Sarcina and Bacillus. Immunological relatedness was compared by Ouchterlony double diffusion experiments and immunoblotting analysis. Immunoblotting is a quantitative technique for measuring levels of specific proteins in crude cell lysates. We have used this technique to measure immunological distance with the assumption that the levels of the various translational components are essentially the same in the different bacterial cells examined. Both immunodiffusion and immunoblotting analysis showed a similar evolutionary relationship between the various species for the two initiation factors examined: (Escherichia = Salmonella greater than Serratia greater than Proteus greater than Aeromonas greater than Pseudomonas). Little or no crossreactivity was found using either analysis with genera: Streptococcus, Sarcina and Bacillus. Using the immunoblot distance, the two initiation factors were shown to diverge at similar rates. One advantage the immunoblotting analysis has over other immunological techniques is that the antigens can be analyzed structurally. We found, for example, that the two forms of IF2 were present in all bacterial species which cross-reacted with anti-IF2, suggesting that both forms are functionally important. Because of its sensitivity, the immunoblot analysis may be more useful than other immunological techniques in studying species that are more distantly related.