Projects . ... SmpB . During trans- translation, stalled bacterial ribosomes are rescued by small protein B ( SmpB ) and by transfer-messenger RNA (tmRNA). ... We present evidences that SmpB binds the large and small ribosomal subunits in vivo and in vitro . ... SmpB contacts the large subunit transiently and early during the trans -translational process. The affinity of SmpB for the two ribosomal subunits is modulated by tmRNA in the course of trans -translation. ...