TRNA(TRP) AS A KEY ELEMENT OF ANTITERMINATION IN THE LACTOCOCCUS-LACTIS TRP OPERON

The expression of the trp operon of Lactococcus lactis is regulated in response to tryptophan availability by a mechanism of transcription a ntitermination, We present evidence in support of a previously describ ed model involving tRNA(Trp) as a key element in the sensing of trypto phan levels and the realization of the regulatory response to tryptoph an limitation, In agreement with this model, two sites of presumed dir ect interaction between the trp leader transcript and tRNA(Trp) are fo und to be of crucial importance for efficient antitermination, These c orrespond to the specifier codon, which presumably interacts with the anticodon in the tRNA, and a sequence complementary to, and presumably interacting with, the acceptor stem of the tRNA, Through these intera ctions, uncharged tRNA(Trp) is believed to stabilize an antiterminator conformation of the trp leader transcript, thus allowing transcriptio n and expression of the structural genes of the operon, For the first time, we present direct evidence that it is the ratio of uncharged to charged tRNA that is important for the regulation of antitermination, rather than the absolute amount of uncharged tRNA, In addition, our re sults indicate that the codon-anticodon interaction, although contribu ting largely to the efficiency of the regulatory response, is not stri ctly indispensable, which suggests the existence of additional interac tions between mRNA and tRNA. Finally, we describe a possible additiona l level of regulation, superimposed and dependent on tRNA-mediated ant itermination control, that is based on the processing of the trp leade r transcript, Together with the regulation mechanisms described earlie r for the Escherichia coli and Bacillus subtilis trp operons, this con stitutes the third different mechanism of transcript elongation contro l found to be involved in the regulation of an operon of which the str uctural genes are highly conserved.