An alternative pathway for 50S assembly in the absence of functional DbpA

DbpA, a DEAD-box RNA helicase, is known to bind helix 92 (H92) of the peptidyl transferase center (PTC) during 50S ribosomal subunit maturation. The presence of H92 and ss/dsRNA junction is essential for its ATP-dependent unwinding activity. However, the precise role of DbpA in 50S precursors (pre50S) and its underlying mechanism remain unclear. In this study, we used cryo-electron microscopy to characterize a series of pre50S intermediates isolated from Escherichia coli cells expressing a catalytically inactive mutant, DbpA(R331A). Pull-down assays revealed that DbpA recognizes only a small subset of pre50S particles in which the central protuberance (CP) is folded, while the PTC remains unfolded. Structural analysis uncovered a series of late-stage intermediates along a progressive assembly pathway, revealing an alternative route for 50S subunit maturation in the presence of non-functional DbpA. In summary, our study provides mechanistic insights into how DbpA facilitates 50S subunit maturation and how the ribosome assembles in its absence.