Regulation of multiple paralogs of a small subunit ribosomal protein in <i>Francisella tularensis</i>

Francisella tularensis is a highly infectious human pathogen that must replicate inside macrophage to cause disease. The ribosomes of F. tularensis can incorporate one of three different paralogs for the small ribosomal subunit protein bS21. One of these paralogs positively impacts translation of key virulence genes and promotes intramacrophage replication. Although ribosomal bS21 content influences F. tularensis virulence, the factors that control bS21 paralog production are not well understood. Here, we reveal that all three bS21 proteins influence the transcript abundance of the paralog important for virulence, bS21-2. In contrast, the other bS21 paralogs (bS21-1 and bS21-3) do not affect their own production. We further determined that the leader sequence of the bS21-2 mRNA is sufficient for bS21-mediated repression of mRNA abundance, suggesting that bS21-2 is autogenously regulated. Counterintuitively, we found that in cells lacking bS21-2, the increase in bS21-2-encoding mRNA does not lead to significant increases in protein production. This reduction in translation efficiency suggests that translation of the bS21-2 mRNA is controlled by other factors. Finally, we determined that bS21-2 exerts at least some of its effects on the bS21-2 transcript by decreasing its stability. Together, our findings suggest that F. tularensis may integrate multiple signals into a regulatory network to control the appropriate production of each bS21 paralog, and particularly the paralog important for virulence, bS21-2. This raises the possibility of a regulatory network that can both control ribosome composition in F. tularensis as well as virulence.