Functional Analysis of BipA in E. coli Reveals the Natural Plasticity of 50S Subunit Assembly

BipA is a conserved translational GTPase of bacteria recently implicated in ribosome biogenesis. Here we show that Escherichia coli ΔbipA cells grown at suboptimal temperature accumulate immature large subunit particles missing several proteins. These include L17 and L17-dependent binders, suggesting that structural block 3 of the subunit folds late in the assembly process. Parallel analysis of the control strain revealed accumulation of nearly identical intermediates, albeit at lower levels, suggesting qualitatively similar routes of assembly. This came as a surprise, because earlier analogous studies of wild-type E. coli showed early binding of L17. Further investigation showed that the main path of 50S assembly differs depending on conditions of growth. Either supplementation of the media with lysine and arginine or suboptimal temperature appears to delay block 3 folding, demonstrating the flexible nature of the assembly process. We also show that the variant BipA-H78A fails to rescue phenotypes of the ΔbipA strain, indicating a critical role for GTP hydrolysis in BipA function. In fact, BipA-H78A confers a dominant negative phenotype in wild-type cells. Controlled production of BipA-H78A causes accumulation of 70S monosomes at the expense of polysomes, suggesting that the growth defect stems from a shutdown of translation.