Synthesis in vitro of ribosomal protein S20 and its precursor.

I have purified and characterized two products synthesized in vitro in a system for coupled transcription and translation programmed by DNA from a transducing bacteriophage carrying the gene for ribosomal protein S20. One of these polypeptides appears to be identical with authentic S20 by several criteria, including its electrophoretic and chromatographic mobilities, and its ability to bind to 16S RNA. The second polypeptide is less basic than S20, but exhibits all the structural and functional properties of a precursor to S20, including the presence of an additional methionine residue, apparently as N-formylmethionine. Moreover, it is converted, albeit slowly, to S20 in cell-free extracts. The persistence of the precursor form of S40 may be functionally significant as well.