[Preparation of Escherichia coli 70S ribosomes labeled with 35S].

[35S]--70S ribosomes (150 Ci/mmol) were isolated from E. coli MRE-600 cells grown on glucose-mineral media in the presence of [35S] ammonium sulfate. The labeled 30S and 50S subunits were obtained from [35S] ribosomes by centrifugation in a sucrose density gradient of 10--30% under dissociating conditions (0.

[Localization of a histidine residue in the binding site for the initiating substrate of E. coli RNA-polymerase].

E. coli RNA polymerase was selectively labelled in the presence of promoters at a histidine residue of the beta-subunit by treatment with GDP beta-imidazolide and then with [alpha-32P]UTP (or [alpha-33P]UTP). Partial cyanogen bromide cleavage of the labelled polypeptide afforded a series of "single-hit" labelled peptides, the electrophoretic pattern of which suggested that the labelling site was His1237.