Noncoordinate translation-level regulation of ribosomal and nonribosomal protein genes in the Escherichia coli trmD operon.

The trmD operon of Escherichia coli contains the genes for the ribosomal protein S16, a 21-kilodalton polypeptide of unknown function, the tRNA(1-methylguanosine)methyltransferase, and the ribosomal protein L19, in that order. As reported elsewhere, the operon is transcribed as a single polycistronic mRNA species, and there is no significant difference in the steady-state amounts of different parts of the mRNA (A.S. Byström, A. von Gabain, and G.R. Björk, submitted for publication). Furthermore, accumulation of all parts of the transcript is altered in a stringently controlled manner upon starvation for valyl-tRNA. Here we show that the rate of synthesis of the trmD operon proteins increased with increasing growth rate and that the amount in steady state, at a specific growth rate (k = 1.0), of the tRNA(1-methylguanosine)methyltransferase was 260 molecules per gene copy, which is about 40 times lower than the amount of the two ribosomal proteins, whereas the 21-kilodalton protein was synthesized to the amount of about 850 molecules per gene copy. The lower steady-state amount of the two nonribosomal proteins was not due to a higher turnover rate. Synthesis of the 21-kilodalton and TrmD proteins responded differently from that of the two ribosomal proteins during conditions which provoked amino acid starvation, although accumulation of the entire mRNA molecule responds similarly to the rate of synthesis of the two ribosomal proteins. We conclude that the observed differential and noncoordinate expression is achieved by regulation at the level of mRNA translation.