Alterations in the two globular domains or in the connecting α-helix of bacterial ribosomal protein L9 induces+1 frameshifts

The ribosomal SOS subunit protein L9, encoded by the gene rplI, is an elongated protein with an a-helix connecting the N- and C-terminal globular domains. We isolated rplI mutants that suppress the + 1 frameshift mutation hisC3072 in Salmonella enterica serovar Typhimurium. These mutants have amino acid substitutions in the N-terminal domain (G24D) or in the C-terminal domain (194S, A102D, G126V, and FIRS) of L9. In addition, different one-base deletions in rplI altered either the final portion of the C terminus or removed the C-terminal domain with or without the connecting a-helix. An alanine-to-proline substitution at position 59 (A59P), which breaks the alpha-helix between the globular domains, induced +1 frameshifting, suggesting that the geometrical relationship between the N and C domains is important to maintain the reading frame. Except for the alterations G126V in the C terminus and A59P in the connecting alpha-helix, our results confirm earlier results obtained by using the phage T4 gene 60-based system to monitor bypassing. The way rplI mutations suppress various frameshift mutations suggests that bypassing of many codons from several takeoff and landing sites occurred instead of a specific frameshift forward at overlapping codons.