# Protein Assistants of Small Ribosomal Subunit Biogenesis in Bacteria
## 1 Introduction  
The ribosome is a large ribonucleoprotein complex responsible for the protein synthesis in all cells. The ribosome comprises small and large subunits containing several ribosomal RNAs (rRNAs) and dozens of proteins (r-proteins). Defects in the ribosome assembly lead to errors in translation and cause severe phenotypes up to cell death [1]. The essentiality of this process makes it a prospective target in the fight against pathogenic bacteria to improve antibacterial therapy. Faulty ribosome biogenesis underlies a spectrum of genetic diseases in humans and is called ribosomopathy [2]. Therefore, ribosome biogenesis is a fundamental process whose accuracy and efficiency is crucial. Ribosome biogenesis is complicated multistage process. The basic steps of ribosome assembly are: (1) the transcription, processing, folding, and modification of rRNA; (2) the translation, folding, and modification of r-proteins; and (3) consecutive binding of ribosomal proteins to rRNAs [3]. All these steps must occur in a certain order. The biogenesis of bacterial ribosomal subunits begins with the transcription of the primary rRNA transcript, which contains the precursors of 16S, 23S, and 5S rRNAs, and some tRNAs (Figure 1) [4, 5]. Precursors of rRNAs and tRNAs fold cotranscriptionally prior to their excision from the primary transcript. Presence of unprocessed 5\({}^{\prime}\)- and 3\({}^{\prime}\)-ends in the rRNA precursors is critical to gain the native structure of ribosomal subunits in vivo. Therefore, the rRNA processing occurs at the final stage of the ribosome biogenesis. However, incorporation of r-proteins and modifications of the r-proteins and rRNAs by specific enzymes occur in concert with the rRNA folding (Figure 1). The r-proteins associate with rRNAs in a hierarchical manner and direct the folding of the rRNAs [6]. During the late assembly steps; for instance, the folding of functional centers of the ribosome, kinetic traps are abundant. The local minima in the folding landscape correspond to erroneous RNA conformations that appear due to degenerate local interactions [7]. At physiological conditions, these conformations can rearrange into the native structure, but rather slowly. Ribosome assembly factors are thought to avoid such kinetic traps, thus allowing the correct ribosome maturation. In other words, ribosome assembly factors limit the folding pathways by assisting proper rRNA folding and protein-RNA interactions [3]. In addition, these factors may sense the checkpoints during the assembly. Ribosome biogenesis factors (also called ribosome assembly factors) are a broad spectrum of proteins that facilitate the formation of functional centers of the ribosome. Assembly factors include GTPases, RNA helicases, endonucleases, modification enzymes, molecular chaperones, etc. [1]. Some ribosome assembly factors have overlapping functions, and such redundancy is believed to be useful for the production of functional and mature ribosomes. Inactivation of these maturation factors leads to the accumulation of unprocessed rRNAs and, as a result, the production of defective ribosomes that cause translational errors and severe phenotypes [1]. Here, we focused on the bacterial ribosome biogenesis factors that assist the production of the small ribosomal subunit (the 30S): RbfA, RimM, RimP, Hfq, Era and YjeQ (RsgA), the methyltransferase KsgA (RsmA), and the acetyltransferase RimJ. These assembly factors allow the functionally important region of the 30S ribosomal subunit--the decoding center (CDR)--to adopt the correct fold. CDR controls the correctness of the codon-anticodon  
Figure 1: Main steps of the small ribosomal subunit (30S) assembly in bacteria. The steps are described in the main text in more detail. Hierarchical incorporation of the ribosomal proteins corresponds to Nomura assembly map [6]. Unprocessed 5\({}^{\prime}\)- and 3\({}^{\prime}\)-ends of the 17S rRNA are shown in red and green, respectively. base pairing [8]. The 30S subunit is composed of one rRNA molecule (16S rRNA) and about 21 r-proteins.