Isolation and molecular genetic characterization of the Bacillus subtilis gene (infB) encoding protein synthesis initiation factor 2.

Western blot (immunoblot) analysis of Bacillus subtilis cell extracts detected two proteins that cross-reacted with monospecific polyclonal antibody raised against Escherichia coli initiation factor 2 alpha (IF2 alpha). Subsequent Southern blot analysis of B. subtilis genomic DNA identified a 1.3-kilobase (kb) HindIII fragment which cross-hybridized with both E. coli and Bacillus stearothermophilus IF2 gene probes. This DNA was cloned from a size-selected B. subtilis plasmid library. The cloned HindIII fragment, which was shown by DNA sequence analysis to encode the N-terminal half of the B. subtilis IF2 protein and 0.2 kb of upstream flanking sequence, was utilized as a homologous probe to clone an overlapping 2.76-kb ClaI chromosomal fragment containing the entire IF2 structural gene. The HindIII fragment was also used as a probe to obtain overlapping clones from a lambda gt11 library which contained additional upstream and downstream flanking sequences. Sequence comparisons between the B. subtilis IF2 gene and the other bacterial homologs from E. coli, B. stearothermophilus, and Streptococcus faecium displayed extensive nucleic acid and protein sequence homologies. The B. subtilis infB gene encodes two proteins, IF2 alpha (78.6 kilodaltons) and IF2 beta (68.2 kilodaltons); both were expressed in B. subtilis and E. coli. These two proteins cross-reacted with antiserum to E. coli IF2 alpha and were able to complement in vivo an E. coli infB gene disruption. Four-factor recombination analysis positioned the infB gene at 145 degrees on the B. subtilis chromosome, between the polC and spcB loci. This location is distinct from those of the other major ribosomal protein and rRNA gene clusters of B. subtilis.