Fragments of ribosomal protein S1 and its mutant form m1-S1. Localization of nucleic-acid-binding domain in the middle region of S1.

Escherichia coli ribosomal protein S1 and its mutant, shorter, form m1-S1 were cleaved at internal methionyl residues to yield, respectively, six and five fragments of Mr ranging from 1000 to 24000. Methods are described to isolate the fragments in pure form. Four of the fragments (designated F2a, F2b, F3 and F4) contain between 86 and 215 amino acids and are therefore as large as other ribosomal proteins. Fragment F2a, derived from the N-terminal region, has previously been shown to contain the major ribosome binding domain of S1 [S. Giorginis and A. R. Subramanian (1980) J. Mol. Biol. 141, 393--408]. Here we show that the RNA binding domain of S1 is essentially contained in F3 derived from the middle region of S1 and carrying the nonreactive--SH group. The reactive--SH group of S1, whose activity is modified by ligand binding, was localized in F2b, a fragment with little RNA binding capacity. The characteristic RNA binding domain and a weak ribosome binding domain of S1 have previously been localized in the large trypsin-resistent core S1-F1 [T. Suryanarayana and A. R. Subramanian (1979) J. Mol. Biol. 127, 41--54]. Together these data indicate that two of the key functional domain of S1 are located in two regions of the molecule separated by an open, exposed segment. The present study also revealed that the nonreactive--SH group of S1 becomes reactive in m1-S1 by the loss of the remote C-terminal region in the latter.