The protein synthesis initiation factor, IF2, in Bacillus subtilis has previously been characterized as being present in two forms, α and β, of molecular mass 79 and 68 kDa, respectively, on the basis of their cross-reaction with anti-E. coli IF2 antibodies and by the DNA sequence of the gene for IF2, infB B.su. In this work we have cloned infB B.su in E. coli cells. Two proteins of molecular mass identical to the B. subtilis IF2α and -β were over-expressed and purified using a new three-step ion-exchange chromatography procedure. The N-terminal amino acid sequence of the two proteins was determined and the results confirmed that the two forms were IF2α and -β, both encoded by the infB gene. The N-terminal amino acid sequence determined for IF2β is Met94-Gln-Asn-Asn-Gln-Phe. The presence of methionine at position 94 shows that this form is, in fact, the result of a second translational initiation in infB B.su mRNA, since the codon at amino acid position 94 is GUG, which is the normal codon for valine, but also known to be an initiator codon. This is a new example of the unusual tandem translation in E. coli of an open mRNA reading frame.
