Translation Inhibition of the Salmonella fliC Gene by the fliC 5′ Untranslated Region, fliC Coding Sequences, and FlgM

The 5′-untranslated region (5′UTR) of thefliC flagellin gene ofSalmonella contains sequences critical for efficientfliC mRNA translation coupled to assembly. In a previous study we used targeted mutagenesis of the 5′ end of thefliC gene to isolate single base changes defective infliC gene translation. This identified a predicted stem-loop structure, SL2, as an effector of normalfliC mRNA translation. A single base change (−38C:U) in thefliC 5′UTR resulted in a mutant that is defective infliC mRNA translation and was chosen for this study. Motile (Mot+ ) revertants of the −38C:T mutant were isolated and characterized, yielding several unexpected results. Second-site suppressors that restoredfliC translation and motility included mutations that disrupt a RNA duplex stem formed between RNA sequences in thefliC 5′UTR SL2 region (including a precise deletion of SL2) and bases early within thefliC -coding region. A stop codon mutation at position 80 offlgM also suppressed the −38C:T motility defect, whileflgM mutants defective in anti-σ28 activity had no effect onfliC translation. One remarkable mutation in thefliC 5′UTR (−15G:A) results in a translation defect by itself but, in combination with the −38C:U mutation, restores normal translation. These results suggests signals intrinsic to thefliC mRNA that have both positive and negative effects onfliC translation involving both RNA structure and interacting proteins.