Purification and properties of two initiation factors from Bacillus stearothermophilus.

Two initiation factors have been isolated from the thermophilic bacterium, Bacillus stearothermophilus, and purified to near homogeneity. The two factors possess physical characteristics and activities associated with the E. coli initiation factors IF-2 and IF-3, and are interchangeable with these factors. The two systems present, however, several differences : S-IF-2 is significantly more heat stable than E. coli IF-2, loosing less than 50 per cent of its activity after 20 minutes at 70degreesC. S-IF-2 alone is unable to promote initiation complex formation on B. stearothermophilus or E. coli ribosomes, and S-IF-3 is absolutely necessary for initiation of complex formation on B. stearothermophilus ribosomes. No factor corresponding to IF-1 has been found. S-IF-3 appears to be able to replace at least partially IF-1, since S-IF-3 and E. coli IF-2 are sufficient to promote maximum fMet-tRNA binding to E. coli ribosomes, while E. coli IF-3 and IF-2 also require IF-1. The differences between the two systems are perhaps required because of the elevated temperature at which B. stearothermophilus normally grows.