## 2 Structure of YchF/Ola1 and Its Interaction with Ribosomes
coli_[20; 36; 37; 42], yeast [41; 45], _Trypanosoma_[38], plants [46; 40] and humans [39] (Table 1). Still, the specific binding site of YchF/Ola1 on the ribosome is unknown. Co-fractionation studies indicate co-migration of YchF/Ola1 with the 30(40)S subunit, the 50(60)S subunit, 70(80)S ribosomes and polysomes [37; 38; 39; 42]. Co-immune precipitation studies in _Trypanosoma_ further indicate that YchF binds to the 60S ribosomal protein uL24 at the ribosomal tunnel exit and to the 40S ribosomal protein eS7, which is implicated in rRNA processing and translational fidelity [38; 47]. In vivo site-directed cross-linking studies confirmed contacts of YchF with both the 30S and the 50S ribosomal subunits in _E. coli_ (Figure 3) [42]. The large majority of these contacts were found on the 30S subunit, e.g., uS5, uS8, uS11 and bS21, which are located at the interface between the 30S and 50S subunits, close to the mRNA entrance site (Figure 3) [42]. A similar contact of Ola1 to the 40S-60S interface is deduced from the observation that the over-production of Ola1 rescues the phenotypes associated with deletions of the ribosomal proteins uS17 and eS7 in yeast [45]. Both proteins are located at the mRNA corridor between the 40S and 60S subunits and are required for translational fidelity [48]. The cross-linking approach in _E. coli_ also detected uL29 as a potential contact site for YchF [42]. Similar to uL24, uL29 is located at the ribosomal tunnel exit of the 50S subunit [49], suggesting that YchF/Ola1 can interact with both subunits of the ribosome. The N-terminal ATPase domain of YchF appears to serve as a major contact site to the ribosome [42], which supports the observation that ribosome binding stimulates ATPase activity [36]. The available data so far do not support the presence of a single binding site for YchF on the ribosome. In _E. coli_, YchF is largely sub-stoichiometric to ribosomes [36; 50] and only approx. 10% of all ribosomes can be in contact with YchF in vivo. Thus, most studies on ribosome binding were performed under YchF over-producing conditions, which likely influenced the YchF-ribosome interaction. This is supported by the observation that in _E. coli_ wild type cells, YchF was exclusively found in the 30S ribosomal fraction, while in YchF-over-producing cells it was found in the 30S, 50S, 70S and polysome fractions [42]. In accordance with the available functional data (see below) the 30S/40S ribosomal subunit seems to be the preferred binding site for YchF/Ola1, while binding to the 50S/60S subunit is probably only transient [36; 38; 39; 42]. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Organism** & \
\begin{tabular}{c} **Ribosome** \\ **Binding** \\ \end{tabular} & **Translation** & **Proteolysis** &
\begin{tabular}{c} **Cellular** \\ **Adaptation**1 \\ \end{tabular} & **References** \\ \hline _Escherichia coli_ & + & + & - & + & [20; 36; 42] \\ _Brucellic bulletins_ & n.a. 2 & n.a. & n.a. & + & [51; 52] \\ _Haempophilus_ & n.a. & n.a. & n.a. & + & [27] \\ _influencea_ & & n.a. & n.a. & + & [53] \\ _Striptococcus_ & n.a. & n.a. & n.a. & + & [54] \\ _Pneumonia_ & & n.a. & n.a. & + & [54] \\ _Vibrio vulnticus_ & n.a. & n.a. & n.a. & + & [54] \\ _Saccharomyces cerevisiae_ & + & + & + & + & [41; 45; 55] \\ _Arabidopsis thaliana_ & + & n.a. & n.a. & + & [22; 40] \\ _Orla sativa_ & + & n.a. & n.a. & + & [22; 46; 56] \\ _Tyngurason cruzi_ & + & n.a. & + & n.a. & [38] \\ _Mus musculus_ & + & + & + & + & [39; 57; 58] \\ _Homo sapiens_ & + & + & + & + & [39; 59; 60] \\ \hline \hline \end{tabular}  
* cellular adaptation includes stress response, metabolic alterations, virulence and other adaptation pathways that are influenced by YchF/Ola1; see text for details. 2 n.a. = not analysed/ no data available.  
\end{table} 
Table 1: Function of YchF/Ola1 in different species. Figure 3: Contact sites for YchF on the _E. coli_ 70S ribosome. Contact sites were determined by site-directed in vivo cross-linking combined with mass spectrometry [42].