Growth-rate-dependent regulation of ribosome synthesis in E. coli: Expression of the lacZ and galK genes fused to ribosomal promoters

Abstract

Hybrid transducing phages were constructed in vitro that carry the galK gene fused to each of three ribosomal promoters: the promoter for an rRNA operon ( rrnE); the promoter for the spc r protein operon; and the promoter for the α r protein operon. We also constructed hybrid transducing phages that carry the lacZ gene fused to the promoter for the rrnE operon or to the promoter for the spc r protein operon. The amounts of galactokinase (or β-galactosidase) were analyzed in lysogens carrying these various transducing phages grown in several different growth media. The synthesis rate of galactokinase (or β-galactosidase) from the fused rrn-gal (or rrn-lac) operon relative to the total protein synthesis rate increased with increasing growth rate, as expected from the transcriptional activity of rRNA operons. In contrast, the relative synthesis rate of galactokinase (or β-galactosidase) from the operon fused to α or spc r protein promoter remained approximately constant with increasing growth rate. These results were interpreted to mean that the characteristic increase in the relative synthesis rate of r protein with increasing growth rate is determined not by transcriptional regulatory mechanisms, but by posttranscriptional mechanisms, which presumably involve the feedback inhibition of r protein mRNA translation by free r proteins.