RNASE-E AUTOREGULATES ITS SYNTHESIS BY CONTROLLING THE DEGRADATION RATE OF ITS OWN MESSENGER-RNA IN ESCHERICHIA-COLI - UNUSUAL SENSITIVITY OF THE RNE TRANSCRIPT TO RNASE-E ACTIVITY

RNase E is a key regulatory enzyme that appears to control the princip al pathway for mRNA degradation in Escherichia coli. Here, we show tha t RNase E represses its own synthesis by reducing the cellular concent ration of the me (RNase E) gene transcript. Autoregulation is achieved by modulating the longevity of this 3.6-kb mRNA, whose half-life rang es from <40 sec to >8 min depending on the level of RNase E activity i n the cell. feedback regulation is mediated in cis by the 5'-terminal 0.44-kb segment of me mRNA, which is sufficient to confer this propert y onto a heterologous transcript to which it is fused. Like the intact protein, an amino-terminal fragment of RNase E lacking 563 amino acid residues can act in trans to repress me gene expression. Paradoxicall y, raising the me gene copy number 21-fold in E. coIi causes an unexpe cted reduction in the concentration of the full-length me transcript, yet results in a small increase in RNase E protein production These su rprising phenomena are explained in terms of a model in which the degr adation of this long and highly labile mRNA commences before elongatio n of the nascent transcript has been completed. In such circumstances, gene expression can be unusually sensitive to changes in mRNA stabili ty.