Control of ribosomal RNA synthesis in Escherichia coli. II. Ribosomal RNA synthesis in isolated nucleoids.

The effect of amino acid-starvation on the transcription in vitro of overall RNA and ribosomal RNA was investigated using nucleoids prepared from the exponentially growing and the amino acid-starved cells of rel+ and rel- strains of Escherichia coli. In this system, the synthesis of RNA is exclusively due to elongation of the chains which have been initiated in vivo. The amounts of overall and ribosomal RNA synthesized per unit of DNA in the nucleoids were analyzed for each preparation. The following observations have been made. (1) The total RNA synthesis per unit of DNA in the nucleoids from the amino acid-starved rel+ and rel- cells was not significantly different from each other. (2) The preferential ribosomal RNA synthesis occurred in the nucleoids from the growing cells; the ribosomal RNA synthesis was restricted in the nucleoids from the starved rel+ cells, while no restriction was observed in the nucleoids from the starved rel- cells. The results suggest that the ribosomal RNA synthesis is regulated at the initiation or less likely elongation level of the transcription. (3) A ribosomal RNA of a discrete size of about 30S was synthesized in the nucleoids. No mature ribosomal RNA species was produced in this system. The 30S RNA is probably a primary transcript of ribosomal RNA genes containing 23S, 16S and 5S mature ribosomal RNA sequences.