# Protein Assistants of Small Ribosomal Subunit Biogenesis in Bacteria
## 2 30S Ribosomal Subunit Biogenesis Factors
### RimM  
Ribosome maturation factor M (RimM) is conserved in bacteria. Rim-like proteins are present in eukaryotes such as _Arabidopsis thaliana_, _Anopheles gambiae_, _Plasmodium falciparum_, and _Plasmodium spoelii_, but were not found in archaea [95]. RimM is a small two-domain protein with a molecular weight of 21 kDa in _E. coli_. Each domain folds into a \(\beta\)-barrel structure (Figure 3f) (PDB ID: 2F1L; 2 QGG; 2 DYI; 3H9N; 3A1P); [96]. The N-terminal part of the protein RimM demonstrated extensive similarity to the KH-domain and contains two copies of the highly conserved GxxG motif involved in interactions with RNA [23]. Due to the structural similarity of the C-terminal domain of RimM to one of the subunits of the photosynthetic reaction center in purple bacteria, it has been called the PRC-barrel domain [97]. This domain is essential for the interaction with the r-protein S19 [95]. [PERSON] and coauthors first observed the ribosome-related function of RimM. They showed that _rimM_ deletion led to slow cell growth and a reduced translational efficiency [98]. Later, they demonstrated that most parts of the 16S rRNA were not processed in cells with _rimM_ deleted [12]. In addition, the _rimM_ deletion aggravated the growth defect of the \(\Delta yieQ\) strain [71]. The genetic interaction between _rimM_ and _yieQ_ was consistent with the hypothesis that RimM plays a role in 30S biogenesis and is related to YieQ's function. At the same time, overexpression of Era is not able to suppress a _rimM_ disruption phenotype. Suppressor mutations in \(\Delta rimM\) and RimM-YY\(\rightarrow\)AA strains were analyzed to study RimM's involvement in 30S subunit maturation [99; 12; 95]. The RimM-YY\(\rightarrow\)AA mutation in the PRC-barrel domain demonstrated reduced binding of RimM to the 30S subunit and accumulation of the 17S rRNA [95; 99]. Suppressor mutations in \(\Delta rimM\) and RimM-YY\(\rightarrow\)AA strains eliminated the deficiency of 16S rRNA processing [95; 12; 99]. In addition, the \(\Delta rimM\) knockout could be compensated by an increased gene dosage of _rbfA_[11]. Suppressor mutations in the \(\Delta rimM\) strain seemed to affect the transcription or stability of the _rbfA_ mRNA. In the RimM-YY\(\rightarrow\)AA strain, suppressor mutations were detected in the _rimM_ mRNA that increased RimM synthesis [99]. Furthermore, in both the \(\Delta rimM\) and RimM-YY\(\rightarrow\)AA strains, suppressor mutations were found in helices 31 and 33b of 16S rRNA, and in genes encoding r-proteins S13 and S19 [95]. These findings suggested that RimM is important for the maturation of the head region of the 30S subunit, which includes r-proteins S13 and S19 and helices 31 and 33b. Direct evidence of RimM participation for the correct structure/conformation of the head domain of the 30S subunit was obtained using in vivo X-ray footprinting and cryo-electron microscopy [20; 21; 32]. Using a synchrotron X-ray beam to generate hydroxyl radicals in the cytoplasm revealed that nucleotides of helices 31 and 33 were exposed in the \(\Delta rimM\) strain [32]. Cryo-EM analysis of \(\Delta rimM\) pre-30S subunits showed a high rotation of the 3'-head domain relative to the body domain; under-representation of head r-proteins S10, S14, S13, and S19; and fragmented densities for helices 44 and 45 of the 16S rRNA [20; 21]. Incubation of \(\Delta rimM\) pre-30S subunits with RimM significantly reduced the flexibility of the head domain. However, helix 44 still displayed a large variability, indicating that RimM probably did not affect the final positioning of the helix 44 in the decoding center [21]. Furthermore, the kinetic data showed that RimM accelerated binding of some head domain r-proteins, S9 and S19, but inhibited interaction with the central pseudoknot-binding protein S12 [100]. Together, these results suggest that RimM does not simply recruit S19 to the complex.