The expression of the operon, which encodes the first specific enzyme of lysine biosynthesis, is controlled by the availability of the end product, lysine. The question of whether lysine exerts its control by inducing premature termination of transcription was addressed using Northern blot analysis. Whereas -specific RNA from lysine-starved consisted primarily of the expected full-length mRNA (1.6 kb), that from bacteria grown with an excess of lysine consisted of a truncated 0.27 kb RNA in place of the full-length 1.6 kb transcript. On the other hand, a mutant, in which the operon was derepressed owing to a single nucleotide substitution in the region corresponding to the leader transcript, produced full-length mRNA, but no 0.27 kb RNA, even during growth with excess lysine. Mapping of the truncated 0.27 kb RNA by hybridization with oligonucleotide probes showed that it corresponded to the upstream portion of the leader transcript, extending from the transcription initiation site to a putative rho-independent terminator element. Quantitative transcript analysis by hybridization with specific oligonucleotides showed that lysine did not affect the number of -specific RNA molecules but promoted the stoichiometric replacement of full-length mRNA with truncated 0.27 kb molecules. These results indicate that lysine regulates the expression of the operon by effecting the premature termination of transcription at a rho-independent terminator site in the leader region and that the site of the mutation, far upstream of the putative terminator element, must play an essential role in premature transcription termination by a mechanism which is not yet understood.


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