LEPA has a special C-terminal domain called CTD with an unusual fold which might interact with tRNA or 23S rRNA. Although the overall structure of LEPA has been described in great detail, the physiological functions involved in translation have not yet been resolved. In E. coli, LEPA is located upstream of the LEP gene, which encodes nonspecific signal peptidase I. Deletion of LEPA does not cause any apparent phenotype under optimal growth conditions. These observations are difficult to reconcile with the ubiquity of LEPA and its extreme conservation. Other results have demonstrated that, although E. coli LEPAdefective cells grown in rich medium have no phenotype, under several stress conditions, including high salt, low pH, and low temperature, the LEPA mutant is overgrown by wild-type bacterial cells. In bacteria, DLEPA strains have been shown to be hypersensitive to potassium tellurite and penicillin and to enhance the production of the calcium-dependent antibiotic in Streptomyces bacteria. Recent studies suggested that LEPA may react with both the PRE and POST ribosome complexes, leading to the formation of an intermediate complex that effectively sequesters a catalytically active ribosome, resulting in a transient inhibition of elongation that provides a mechanism for the optimization of functional protein synthesis. A slightly high chlorophyll fluorescence and pale green phenotype are detected in the cplepa-1mutant when grown under normal growth conditions. Physiological and biochemical analyses of the mutant revealed that cpLEPA has an important function in SCH727965 chloroplast biogenesis and plays an essential role in chloroplast translation. LEPA is an extremely conserved and widely distributed translation factor. The amino acid sequence of Arabidopsis cpLEPA shows 64% amino acid identity with that of E. coli LEPA. This degree of sequence conservation is particularly high for a comparison between higher plants and bacteria. CpLEPA contains four domains: LEPA, LEPA-II, LEPA-C and a CTD domain. The LEPA and LEPA-II domains contain the extremely conserved key amino acids that are important for GTP binding, which are known as the G1, G2, G3 and G4 sequence motifs. The G1, G3 and G4 motifs are responsible for binding and hydrolyzing GTP and for interacting with the cofactor Mg2+. The G2 motif undergoes a conformational change that is essential for GTPase function. LEPA-C was predicted to function in translation elongation. The structure and sequence similarity of cpLEPA to E. coli LEPA indicates a role for this protein in the efficiency of chloroplast protein translation. LEPA was initially reported as the leader peptide of the lep operon and was described as a membrane-associated GTPbinding protein. The N-terminal 51 amino acids of Arabidopsis cpLEPA was hypothesized to function as a chloroplast signal peptide. Membrane-associated cpLEPA could be washed out by Na2CO3 and CaCl2, indicating that cpLEPA is not an integral membrane protein and that the association with the membrane is flexible. Pech et al suggested that the membrane acts as a storage depot for LEPA and that LEPA is released into the cytoplasm as needed under specific stress conditions in E. coli. Considering the association of cpLEPA with the thylakoid membrane, such an arrangement might facilitate the production of functional protein under different stress conditions. We also observed no growth differences between the cplepa-1 mutants and wild-type plants when grown on MS medium supplied with 2% sucrose under a light intensity of 120 mmol m22 s 21. However, the growth of cplepa-1 was greatly retarded on MS medium supplied with 1% sucrose or without sucrose under the same light intensity.