The regulation of gene expression was studied, for the Escherichia coli rpoBC 
operon, which includes the genes, rpoB and rpoC, for the beta and beta subunits 
of RNA polymerase, and rplJ and rplL, for the two proteins, L10 and L7/12, of 
the 50S ribosome. The gene organization agrees well with the accumulated 
observations indicating the coordinate synthesis of RNA polymerase and ribosomes 
under various growth conditions for wild-type E. coli cells. On the other hand, 
the differential regulation of the two essential components observed under 
restrictive growth conditions, after addition of various drugs or with certain 
mutants, in particular those carrying mutations in the RNA polymerase genes, was 
found to take place through two novel regulation systems: The transcriptional 
termination at an internal attenuation site and the two autogenous and 
posttranscriptional controls, being specific for the two ribosomal protein genes 
and the two RNA polymerase subunit genes, respectively. The majority of the 
transcription initiated from the promoter rpoP beta terminates at an attenuator 
site between the promoter-proximal rplJL and the promoter-distal rpoBC genes. 
The frequency of the attenuation seems to control the relative level of RNA 
polymerase synthesis to that of ribosomes. The expression of rpoBC genes is 
subject to an autogenous regulation, in which both RNA polymerase holoenzyme and 
alpha 2 beta complex function as regulatory molecules with repressor activity. 
The autogenous regulation was found to operate at post-transcriptional step(s), 
probably at the level of translation. During the study on the regulation of RNA 
polymerase synthesis, we noticed that the rpoBC operon contained another 
autogenous regulation circuit, in which the synthesis of L10 and L7/12 was 
specifically repressed by the L10-L7/12 complex. Molecular mechanisms and 
physiological meanings of the novel regulations are discussed.