# Protein Assistants of Small Ribosomal Subunit Biogenesis in Bacteria
## 2 30S Ribosomal Subunit Biogenesis Factors
### Network of 30S Assembly Factors
Effects of the overproduction of one assembly factor on the defects caused by the lack of another one are shown by arrows. A plus sign indicates a positive effect, a minus sign indicates a negative effect, and a cross (\(\times\)) signifies that no effects were observed. A positive effect implies the partial suppression of the defects of ribosome assembly. A negative effect implies the exacerbation of the defects of ribosome assembly. This network is described in the main text in more detail. rRNA. RbfA is thought to refold the 5'-leader sequence via h28 and to promote formation of h1. It may also act synergistically with RimP to stabilize the central PK. RimP plays a key role in the central PK stabilization, accelerating the subsequent incorporation of central PK-binding r-proteins S5 and S12. Obviously, RimJ acetylates S5 at this stage, signaling that the central pseudoknot is formed. RimJ may dissociate after acetylation. We cannot rule out that RimJ is also directly involved in the central PK stabilization--the remodeling the structure of the central pseudoknot for stable association of protein S5 with its subsequent acetylation. Then, the formed central pseudoknot anchors the head domain to the body and platform. At that point, docking of the head to the platform domain may occur either with an assembled head domain or with one only partially formed. Therefore, the RimM-promoted folding of the 3' major domain and RbfA-supported conformational changes in the 5' domain required for the formation of the central pseudoknot may occur additively. However, dissociation of RimM occurs prior to stabilization of the central PK incorporation of S12. RimP appears to remain on the pre-30S subunit until the final stages of the 17S rRNA processing. It binds to S12 so that h44 cannot access its native position, keeping the upper part of h44 and its linker regions disordered. Thus, RimP maintains the mRNA channel unstructured in the neck region for the rearrangement of the 3'-end region of the 17S rRNA. It has been considered that RimP exposes the KsgA binding site and may even preassociate with KsgA to facilitate its recruitment to the subunit. The checkpoint proteins KsgA and YjeQ are involved in the CDR maturation after the formation of the central PK and docking of the head to the platform domain. YjeQ binds to the head and neck of h28 and the top of helix 44 and probes the state of maturating 30S subunits. YjeQ induces release of RbfA. As a result, the 3'-end region of the 17S rRNA is free to rearrange near the neck and interact with h28 to stabilize the h44/45 linker. YjeQ further promotes h44/45 linker maturation. It clamps around the top of the h44, fixing the positions of A1492, which monitors the geometry of the codon-anticodon base pairing during the translation, and of the adjacent adenosine A1493. Limiting the h44/45 linker-folding landscape favors interactions with the minor groove of h44. After that, the GTP-activity is triggered, and YjeQ dissociates from the pre-30S subunit. Then, quality of the CDR assembly is monitored by the methyltransferase KsgA, keeping h44 from containing the h45 tetraloo. Dimethylation of A1518 and A1519 induces the packing interaction between helices 45 and 44. The methylation is assumed to be a trigger for the release of KsgA from assembling 30S subunits. Most likely, the release of KsgA is accompanied by Era dissociation and RimP exchange for Hfq, which are required for the final stages of the 17S rRNA processing. Hfq, along with RNAses, form the productive structure of the 5'- and 3'-ends of the16S rRNA. The final h44/h45 docking on the front of the 30S subunit occurs after the trimming of the 5'- and 3'-ends of the 17S rRNA and the release of Hfq/RNAes. This allows the 30S subunit's entrance into the translation cycle.