- Cell-free arabinose binding protein (ABP) was synthesized using a mRNA-directed Escherichia coli S30 translation system. The source of the mRNA was a total cellular RNA extract from cultures of E. coli B/r ara A 39, induced for ABP production. Purification of in vitro ABP was effected by affinity chromatography on a column of purified anti-ABP coupled to Sepharose 4B, followed by Sephadex G-75 chromatography in 9% formic acid. The purified in vitro ABP was found to have a molecular weight approximately 3000 greater than native ABP. Comparison of the CNBr peptide fragments of native and in vitro ABP demonstrated an NH2-terminal extension of 23 amino acids not present in native ABP. The identities of 20 of the residues in the extension were established, and the characteristics of this region resemble the features proposed for signal sequences that function in protein secretion.