# Protein Assistants of Small Ribosomal Subunit Biogenesis in Bacteria
## 2 30S Ribosomal Subunit Biogenesis Factors
### YjeQ (RsgA)  
YjeQ is a ribosome-dependent GTPase (also known RsgA-Ribosomal small subunit **G**T**parse **A**), the activity of which is stimulated by the mature 30S subunit [33; 34; 35] and inhibited by the stringent response mediators pppGpp and ppGpp [27; 28]. The ribosome assembly factor YjeQ is found in Gram-negative bacteria, and its homolog YloQ is present in Gram-positive bacteria. There are no YjeQ/YloQ counterparts in eukaryotes and in archaea. The _yjeQ_ gene is important for bacterial growth [36]. The bacterial cells with a knockout of the _yjeQ_ or _yloQ_ gene are viable but grow very slowly and accumulate the 17S rRNA [19; 37; 38]. YjeQ (about 33 kDa in _Thermotoga maritima_) consists of three domains (Figure 3b) [39; 40; 41; 42]. The N-terminal or OB-fold domain (oligonucleotide/oligosaccharide-binding) represents a \(\beta\)-barrel. N-terminal amino acid residues from 1 to 20 are essential for tight binding to the 30S ribosomal subunit [33]. The central GTPase domain is a Rossmann fold with a unique circular permutation: instead of the standard order G1-G2-G3-G4-G5, which is characteristic of TRAFAC (**TRAIN**slation **FAC**tor) GTPases, it possesses a G4-G5-G1-G2-G3 motif because the N-terminal and C-terminal loops are swapped [39; 40; 41; 42]. As a result of this rearrangement, the C-terminal domain may have a direct mechanical effect on the GTP hydrolysis in YjeQ. The C-terminal part consists of one 3\({}_{10}\)-helix and the two \(\alpha\)-helices with the zinc-finger motif Cx(F/Y)xxCxHxx(E/D)xxC. [39; 40; 41; 42]. These cysteines and histidine are located in the loop connecting two \(\alpha\)-helices and coordinate a zinc ion. The zinc finger contributes to the structural stability of the C-terminal domain. Cryo-EM analysis of immature 30S particles from the \(\Delta y\)_ieQ_ cells showed that late assembly r-proteins S2 and S21 were underrepresented [18; 19; 20]. Helix 44 of the 16S rRNA was distorted, and the extent of the distortion varied from class to class of the \(\Delta y\)_ieQ_ 30S assembly intermediates. The class with the largest h44 distortion showed outward displacement of the upper part of helix 44 [19]. On the other hand, the class with fewer h44 distortions was correlated with the increase in S2 density amount in the cryo-EM maps, indicating that the decoding center was closer to the one found in mature 30S subunits. Most likely, YjeQ is involved in the positioning of the helix 44 within the decoding center at a late stage of the 30S assembly. As for the YjeQ binding site on the ribosome, there are currently five cryo-EM structures of the mature 30S subunit with YjeQ [30; 43; 44; 45; 46]. The first two structures placed YjeQ  
Figure 4: Cryo-EM structures of 30S subunits in complex with different ribosome biogenesis factors: (**a**,**b**) 30**5***RbfA (EMDB ID: 1413; PDB ID: 2R1C and EMDB ID: 12243; and PDB ID: 7 BOH, respectively); (**c**) 30**5***YjeQ (RøgA) (EMDB ID: 12245; PDB ID: 7 NAR); (**d**) 30**5***Era (EMDB ID: 0484); (**e**) 30**5***KøgA (RømA) (EMDB ID: 2017; PDB ID: 4 ADV); (**f**) 30**5***RimP (EMDB ID: 12247; PDB ID: 7 NAT). at the same site on the 30S subunit, but in the opposite orientations [43; 44]. All subsequent 30S subunit\(\bullet\)YjeQ structures confirmed only one of the two orientations of YjeQ (Figure 4c) [30; 45; 46]. The C-terminal domain of YjeQ forms a bridge between the head (h29 and h31) and the platform of the 30S subunit. The N-terminal OB-fold part of YjeQ interacts with h18 and h44 of the decoding center, while the GTPase domain contacts h44 and h24 of the platform. Both GTPase and OB-fold domains of YieQ resemble a clamp around the top of helix 44, limiting its mobility. A1492, along with the other universally conserved A1493, monitors the geometry of the codon-anticodon base pairing in the decoding center, and bulges out from the helix 44. This conformation of A1492 is similar to that adopted by A1492 and A1493 during proofreading of the codon-anticodon pairing [47].