Overexpression of RbfA in the absence of the KsgA checkpoint results in impaired translation initiation

Summary KsgA , a universally conserved small ribosomal subunit ( SSU ) rRNA methyltransferase, has recently been shown to facilitate a checkpoint within the ribosome maturation pathway. Under standard growth conditions removal of the KsgA checkpoint has a subtle impact on cell growth; yet, upon overexpresssion of RbfA , a ribosome maturation factor, KsgA becomes essential. Our results demonstrate the requirement of KsgA , in the presence of excess RbfA , both for the incorporation of ribosomal protein S21 to the developing SSU , and for final maturation of SSU rRNA . Also, when SSU biogenesis is perturbed by an imbalance in KsgA and RbfA , a population of 70 S ‐like particles accumulates that is compositionally, functionally and structurally distinct from mature 70 S ribosomes. Thus, our work suggests that KsgA and RbfA function together and are required for SSU maturation, and that additional checkpoints likely act to modulate malfunctional 70 S particle formation in vivo .