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Regulation of Ribonucleic Acid Synthesis in Escherichia coli During Diauxie Lag: Accumulation of Heterogeneous Ribonucleic Acid

Lewis A. Jacobson
1970 Journal of Bacteriology  
The synthesis of ribonucleic acid (RNA) and of protein in Escherichia coli during glucose-lactose diauxie lag have been examined. The rate of RNA synthesis is about 7%, of the corresponding rate during exponential growth and the rate of protein synthesis 10 to 15%. Inhibition of RNA synthesis occurs to the same extent in both rel and rel+ strains. The RNA which accumulates during 20 min in diauxie lag is composed of about 50% ribosomal and transfer RNA species and about 50% of a fraction which
more » ... f a fraction which resembles messenger RNA (mRNA) in its heterogeneous sedimentation properties. Decay of the heterogeneous fraction occurs in the presence of glucose and actinomycin D with a half-life of 3 min, the same as that of pulse-labeled mRNA; however, during the diauxie lag, the half-life of this RNA is about 25 min. Accumulation of the heterogeneous RNA is further increased when protein synthesis is blocked by chloramphenicol. The data suggest that the disproportionate accumulation of mRNA during diauxie lag and energy source shift-down may be attributed at least in part to increased stability of mRNA, but do not rule out a preferential synthesis of mRNA. The synthesis of ribonucleic acid (RNA) in Escherichia coli is known to be controlled by a number of distinct mechanisms. The formation of specific messenger RNA (mRNA) species is regulated by specific cytoplasmic repressors or activators, whereas the synthesis of ribosomal and transfer RNA species is controlled by the cellular amino acid supply through the rel locus (2). In strains carrying the "stringent" (rel+) allele, RNA accumulation is halted by any condition producing a deficiency of one or more amino acids. RNA synthesis continues under these conditions when the "relaxed" (rel) allele is present. Recent information (8, 11, 16) suggests that amino acid control does not affect the synthesis of mRNA. Regulation of RNA synthesis by carbon or nitrogen source seems to involve an independent mechanism, insofar as it is indifferent to the state of the rel locus (14). When E. coli is shifted to a poorer carbon or nitrogen source a transient inhibition of RNA synthesis ensues (4). Conversely a shift to a medium which will support a higher growth rate evokes overproduction of RNA until the RNA content characteristic of the new growth rate is attained (4). Little is known about the mechanism of this growth rate regulation. The growth lag ("diauxie lag") which E. coli experiences between exhaustion of a source of glucose and the beginning of growth on lactose may be viewed as a form of energy source shiftdown. This case is of particular interest for the regulation of RNA synthesis since it is known that at least one specific mRNA, that homologous to the Lac operon, is actively synthesized during the lag. Indeed, Naono et al. (13) estimated that 20% of all rapidly labeled RNA formed during this period was homologous to the Lac operon. They called this phenomenon "preferential transcription," implying either an abnormally high rate of transcrjption of Lac or a normal transcription of Lac, whereas other regions of the genome are transcribed at a subnormal rate. In this study, the synthesis and accumulation of the various classes of RNA during diauxie lag are examined. The results indicate that the synthesis of gross RNA is severely depressed during diauxie lag, but that not all classes of RNA are 740
doi:10.1128/jb.102.3.740-746.1970 fatcat:zzktphmjffct7cguv2vkju363q