Evidence for a precursor-product relationship in the biosynthesis of ribosomal protein S20

The kinetics of labeling ribosomal protein S20 of Escherichia coli strains H882 and H882 groE44 have been examined using partial reconstitution as a means of binding this and some other 30S subunit proteins selectively to 16S RNA from crude extracts prepared by acetic acid extraction of pulse-labeled whole cells. The rate of labeling of S20 during short pulses at 44 degrees C is less than 20% of that observed at 28 degrees C. S20 can be recovered from the cells labeled at the higher temperature if they are chased at 28 degrees C, but not at 44 degrees C, in the presence of excess sulfate prior to their extraction. These observations suggest that S20 is derived from a precursor whose processing is blocked at 44 degrees C. Among the proteins extracted from cells labeled at 44 degrees C capable of binding to 16S RNA is a novel polypeptide, p2, which is not normally present on the 30S subunit. The kinetics of its appearance at 44 degrees C, and its chasing at 28 degrees C, suggest a precursor-product relationship with S20. p2 contains a tryptic peptide with the chromatographic properties of the peptide Ser-Met-Met-Arg at position 25-28 in S20. A second methionine-containing peptide at positions 49-59 of S20 is missing from p2. In addition, the apparent molecular weight of p2 (8600) is less than that of S20 (9500). p2 may represent the product of degradation of a precursor to S20, yet retains the ability to bind to 16S RNA. It is much less likely that p2 is a bona fide precursor which is converted into S20 by fusion to some other polypeptide.