**1. Introduction**

The *lrtA* gene from *Synechococcus* sp. PCC702 is known to express a light-repressed protein [1,2]. Further investigations have shown that LrtA is involved in the stabilization the 70S ribosomal particles [3], as well as in cell survival during stress circumstances. LrtA is related with other proteins that take part in ribosome activity. Under environmental stress conditions, protein synthesis is stopped in a down-regulation process. Reduction of protein production involves: (i) Formation of the inactive 100S disome through dimerization of 70S particles [4], implicating the action of some proteins; or (ii) protein-ribosome interactions which involve the canonical ribosomal proteins [5,6]. The family of ribosomal proteins in *E. coli* includes YfiA (also known as PY or RaiA, ribosome associated inhibitor A); and YhbH (also known as HPF, hibernation promoting factor). YfiA likely inhibits translation indirectly, involving 70S particles [7,8]. Alternatively, HPF stops translation by stabilizing 100S dimers [8–10]. Most bacteria have one or more homologues related to HPF or YfiA [10]. These homologues can be classified in long HPF, short HPF and YFiA, on the basis of the length of their sequences and the presence of a specific domain. The conserved domain in all of them has the β-α-β-β-β-α fold [5,11], with a β-sheet formed by four strands and two α-helices packed against it. According to its sequence,

LrtA from *Synechocystis* sp. PCC 6803 belongs to the long HPF sub-family. We have previously shown that LrtA is involved in self-association equilibria [12], and has chameleonic structural properties. In particular, molecular dynamics (MD) simulations and experimental analyses suggest that the whole LrtA has a solvent-dependent conformation, where the N terminus adopts the β-α-β-β-β-α fold and the C terminus is disordered and compact [12].

In this work, we have studied the conformational preferences of the isolated C-terminal region of LrtA (residues 102-191), C-LrtA. We aimed to test whether: (i) C-LrtA was disordered and collapsed, as suggested by previous MD simulations of the whole LrtA; and (ii) isolated C-LrtA was oligomeric in solution. Characterizing the degree of disorder in proteins or protein domains, and whether this contributes to attaining a quaternary structure, is important to explain their functions; in fact, most of the intrinsically disordered proteins (IDPs) characterized so far are involved in protein-protein contacts [13], and it is essential to establish how specificity is achieved in those interactions. We show here that C-LrtA was disordered and with a strong self-association tendency, as shown by several biochemical, biophysical and hydrodynamic techniques: Blue-native gels, glutaraldehyde cross-linking, iodide quenching, small-angle X-ray scattering (SAXS), size exclusion chromatography (SEC) and isothermal titration calorimetry (ITC). MD simulations of isolated C-LrtA also predicted a disordered conformation, in reasonable agreement with the experiments. Therefore, we proved that: (i) former MD predictions on C-LrtA, based on the whole parental LrtA, were correct; (ii) the isolated domain has a tendency to self-associate; and (iii) the presence of quaternary interactions in C-LrtA did not induce any stable secondary nor tertiary structure, and therefore, C-LrtA was an oligomeric IDP.
