Background: Ribosomes in Escherichia coli change their composition and conformation in the stationary phase. Ribosome modulation factor (RMF) and ribosomal protein S22 are known to be associated with stationary phase ribosomes. RMF association causes the loss of translational activity and the dimerization of 70S ribosomes into 100S ribosomes, which may increase cell survival in the stationary phase.

During the stationary phase of growth in Escherichia coli, ribosome modulation factor (RMF) and hibernation promoting factor (HPF) dimerize most 70S ribosomes to form 100S ribosomes. The process of 100S formation has been termed 'ribosomal hibernation'. Here, the contributions of HPF to 100S formation and translation were analysed in vitro. HPF bound to, but did not dimerize the 70S ribosome. RMF dimerized and formed immature 90S ribosomes. Binding of both HPF and RMF converted 90S ribosomes to mature 100S ribosomes, which is consistent with the in vivo data. The role of HPF in in vitro translation also was investigated. In an artificial mRNA poly (U)-dependent phenylalanine incorporation assay, HPF bound to ribosomal particles and inhibited translation. In contrast, in a natural MS2 mRNA-dependent leucine incorporation assay, bound HPF was removed and hardly inhibited normal translation. Multiple alignment and phylogenetic analyses indicates that the hibernation system mediated by the HPF homologue, RMF and 100S ribosome formation may be specific to the proteobacteria gamma group. In contrast, most bacteria have at least one HPF homologue, and these homologues can be classified into three types, long HPF, short HPF and YfiA.

F1-adenosine triphosphatase (ATPase) is an ATP-driven rotary molecular motor in which the central gamma subunit rotates inside a cylinder made of three alpha and three beta subunits alternately arranged. The rotor shaft, an antiparallel alpha-helical coiled coil of the amino and carboxyl termini of the gamma subunit, deeply penetrates the central cavity of the stator cylinder. We truncated the shaft step by step until the remaining rotor head would be outside the cavity and simply sat on the concave entrance of the stator orifice. All truncation mutants rotated in the correct direction, implying torque generation, although the average rotary speeds were low and short mutants exhibited moments of irregular motion. Neither a fixed pivot nor a rigid axle was needed for rotation of F1-ATPase.

Ribosomal P0, P1, and P2 proteins, together with the conserved domain of 28 S rRNA, constitute a major part of the GTPase-associated center in eukaryotic ribosomes. We investigated the mode of assembly in vitro by using various truncation mutants of silkworm P0. When compared with wild type (WT)-P0, the C-terminal truncation mutants C⌬65 and C⌬81 showed markedly reduced binding ability to P1 and P2, which was offset by the addition of an rRNA fragment covering the P0⅐P1-P2 binding site. The mutant C⌬107 lost the P1/P2 binding activity, whereas it retained the rRNA binding. In contrast, the N-terminal truncation mutants N⌬21-N⌬92 completely lost the rRNA binding, although they retained P1/P2 binding capability, implying an essential role of the N terminus of P0 for rRNA binding. The P0 mutants N⌬6, N⌬14, and C⌬18-C⌬81, together with P1/P2 and eL12, bound to the Escherichia coli core 50 S subunits deficient in L10⅐L7/L12 complex and L11. Analysis of incorporation of 32 P-labeled P1/P2 into the 50 S subunits with WT-P0 and C⌬81 by sedimentation analysis indicated that WT-P0 bound two copies of P1 and P2, but C⌬81 bound only one copy each. The hybrid ribosome with C⌬81 that appears to contain one P1-P2 heterodimer retained lower but considerable activities dependent on eukaryotic elongation factors. These results suggested that two P1-P2 dimers bind to close but separate regions on the C-terminal half of P0. The results were further confirmed by binding experiments using chimeric P0 mutants in which the C-terminal 81 or 107 amino acids were replaced with the homologous sequences of the archaebacterial P0.The ribosomal large subunits from all organisms contain an active site termed the "GTPase-associated center" that is responsible for the GTPase-related events in protein biosynthesis. This active site is composed of the two highly conserved domains around 1070 and 2660 (Escherichia coli numbering is used throughout) of 23 S/28 S rRNA and the ribosomal proteins bound to the 1070 region (1-3). The protein components of this site in prokaryotic ribosomes constitute a characteristic pentameric complex, L10(L7/L12) 2 (L7/L12) 2 (4, 5), in which two L7/L12 homodimers bind to the C-terminal regions of L10 (6) and constitute a highly flexible and functionally important lateral protuberance, the so-called "stalk" (7). Although the ribosomal stalk is observed by cryo-electron microscopy (8), the detailed structure of this pentameric complex has not been resolved by x-ray crystallography of ribosomes (9 -11). The chemical features of protein-protein and proteinrRNA interactions in the GTPase-associated center remain to be clarified.The animal ribosomal phosphoproteins P0 and P1/P2 (P proteins) are counterparts of prokaryotic L10 and L7/L12, respectively, although P1 and P2 are related but different proteins, unlike prokaryotic L7/L12 (12-14). In yeast cells, there are two P1-type proteins, P1␣ and P1␤, and two P2-type proteins, P2␣ and P2␤ (15). It is believed that P proteins constitute a pentameric complex, designated here as...

During the stationary growth phase, Escherichia coli 70S ribosomes are converted to 100S ribosomes, and translational activity is lost. This conversion is caused by the binding of the ribosome modulation factor (RMF) to 70S ribosomes. In order to elucidate the mechanisms by which 100S ribosomes form and translational inactivation occurs, the shape of the 100S ribosome and the RMF ribosomal binding site were investigated by electron microscopy and protein-protein cross-linking, respectively. We show that (i) the 100S ribosome is formed by the dimerization of two 70S ribosomes mediated by face-to-face contacts between their constituent 30S subunits, and (ii) RMF binds near the ribosomal proteins S13, L13, and L2. The positions of these proteins indicate that the RMF binding site is near the peptidyl transferase center or the P site (peptidyl-tRNA binding site). These observations are consistent with the translational inactivation of the ribosome by RMF binding. After the "Recycling" stage, ribosomes can readily proceed to the "Initiation" stage during exponential growth, but during stationary phase, the majority of 70S ribosomes are stored as 100S ribosomes and are translationally inactive. We suggest that this conversion of 70S to 100S ribosomes represents a newly identified stage of the ribosomal cycle in stationary phase cells, and we have termed it the "Hibernation" stage.