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.