Synthesis and decay of messenger ribonucleic acid from the lactose operon of Escherichia coli during amino-acid starvation

Abstract

Synthesis and decay of messenger RNA from the lac operon of Escherichia coli were studied during amino-acid starvation and normal growth in suA + suA − and rel + rel − strains. At least 35% of the β-galactosidase message length is transcribed during leucine starvation, but there is no evidence that full-length molecules are made during either leucine or tryptophan deprival. In contrast, trp operon mRNA is completed during the same starvations. In both a stringent or relaxed strain βG message is inactivated at the same rates during starvation and normal growth. In normal growth the suA product does not affect the inactivation rates of either βG or transacetylase messages or the chemical loss of lac or total mRNA. However, it does account for 20 to 40% of their decay during leucine starvation. During growth TA message is inactivated two or three times faster than βG message. In starvation TA message is not made, but capacity to complete it and to make TA can be measured after re-addition of leucine. This capacity is lost at a much slower rate than during normal growth and at approximately the same rate as is capacity for βG synthesis. These results suggest: (i) a coupling of transcription to translation in the lac operon during amino-acid starvation, (ii) common targets for loss of capacity of both βG and TA messages on the operator-proximal part of lac mRNA, (iii) a role of the suA product in decay of lac mRNA during starvation and (iv) close to normal decay rates of the incomplete messages even though peptide synthesis is only 7% of normal.