In vivo incorporation of the uridine-photoactivable analogue, 4-thiouridine, into the ribosomal RNA of an Escherichia coli pyrD strain has been demonstrated. It is highly dependent on the exogeneous uridine and 4-thiouridine concentrations as well as on temperature. We have defined conditions allowing the substitution of 13 ± 2% of the uridine residues in bulk RNA by 4-thiouridine. On a high-Mg2+ sucrose gradient, 33 ± 3% of ribonucleic particles sediment as 70S ribosomes, the remaining being in the form of non-associated 50S and 30S particles containing immature rRNA. The thiolated 70S ribosomes tolerate a 4–5% substitution level (40 thiouridine molecules/particle).
Surprisingly, 3–4% of ribosomal proteins, about two protein molecules/particle, were spontaneously covalently bound to 4-thiouridine-substituted rRNA. Specific 366-nm photoactivation increased this proportion to 10–12%, i.e. up to six or seven ribosomal protein molecules/particle. The photochemical cross-linking proceeds with aparent first-order kinetics with a quantum yield close to 5 × 10–3. Although extensive photodynamic breakage of rRNA occurs under aerobic conditions, both the kinetics and yield of ribosomal protein cross-linking were independent of oxygenation conditions. The thiolated (4.5%) 70S ribosomes allowed the poly(U)-directed poly(Phe)synthesis at 48% the control rate. Photoactivation decreased this activity to 28% and 10% when performed under nitrogen and in aerated conditions, respectively.