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Abstract:
Nascent proteins emerging from translating ribosomes in bacteria are screened by ribosome-associated protein biogenesis factors (RPBs), among them the signal recognition particle (SRP), trigger factor (TF), peptide deformylase (PDF), and methionine aminopeptidase (MAP). Whereas SRP and TF can bind to translating ribosomes simultaneously, the time of action for the modifying enzymes, PDF and MAP, is not clear. Here we examine the RPB choreography by monitoring FRET between labeled ribosomes and either TF or SRP. The competition binding analysis reveals how the RPB binding pattern changes depending on the state of the ribosome. We observe that TF, like SRP, rapidly scans all ribosomes until it is stabilized on ribosomes presenting TF-specific nascent chains. PDF and TF compete for binding to those complexes, whereas MAP can bind together with TF. The same competition pattern we observe with SRP and PDF or SRP and MAP on ribosomes exposing an SRP-specific nascent chain. These results suggest that PDF and TF act sequentially in that PDF deformylates short nascent chains prior to stable TF binding to ribosomes carrying longer nascent chains. By contrast, competition between PDF and SRP, which is recruited to ribosomes carrying rather short nascent chains, may interfere with N-terminal processing of nascent membrane proteins that enter the membrane co-translationally in an SRP-dependent fashion.
