Protein synthesis initiation factors from a tetracycline-producing strain of Streptomyces aureofaciens were purified and characterized. Two forms of IF3 (M(r) = 24,000 and 22,500) were found. By Western blot analysis, only one form of protein IF2 cross-reactive with anti-IF2 of Escherichia coli was revealed. The molecular mass of purified IF2 was 69,000 as determined by SDS-polyacrylamide gel electrophoresis. In spite of differences in molecular mass between the IF2 forms of E. coli and the factor from S. aureofaciens, the latter could substitute IF2 of E. coli in the stimulation of codon-specific binding of initiator tRNA. In contrast to the reported absence of IF1 in some Gram-positive microorganisms, we found "protein IF1" (M(r) = 9,000) in S. aureofaciens that increased the IF2-dependent binding of initiator tRNA to ribosomes.

Institute of Microbiology Academy of Sciences of the Czech Republic Prague

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General and molecular microbiology and microbial genetics in the IM CAS