Function of the repeating homologous sequences in nucleic acid binding domain of ribosomal protein S1

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Abstract
Ribosomal protein S1 contains in its RNA binding domain 4 repeating, homologous stretches of sequences. Its functionally active mutant form m1-S1 contains only 3 repeating stretches. In order to assess the functional importance of this repeating sequence, S1 was cleaved at its reactive SH group on Cys-349 and a fragment (S1-F4) that has lost 2 of the homologous stretches but retains all other essential elements was isolated. Ribosomes reconstituted with S1-F4 instead of S1 are functionally active in translating poly(U) and poly(A), but totally inactive in translating phage MS2 RNA. The significance of this result is discussed vis-a-vis the initiation step in translating natural mRNA and a functional role for the tetrarepeat of S1 is suggested. [Escherichia coli was used.].