Novel form of transcription attenuation regulates expression the Bacillus subtilis tryptophan operon

Transcription of the trp operon of Bacillus subtilis is regulated in response to the availability of tryptophan. The first structural gene of the operon is preceded by a 204-base-pair transcribed leader region that contains a segment with the features of a procaryotic termination site. Transcription of the leader region was analyzed in vivo and in vitro to determine whether this putative termination site was used to regulate operon expression. When RNA was isolated from wild-type cells grown in the presence of excess tryptophan, transcripts of the operon ended at the putative termination site. In contrast, RNA isolated from cells grown in the absence of tryptophan or from a mutant strain which is constitutive for trp operon expression contained trp transcripts that extended beyond the termination site into the structural genes. To assess termination quantitatively in vivo, a trpE-lacZ fusion was constructed in which the trp promoter and leader region controls hybrid beta-galactosidase formation. The effects on hybrid beta-galactosidase levels of point mutations and deletions introduced into this leader region were determined. The results obtained establish that transcription of the trp operon structural genes is regulated in the leader region. This regulation appears to be mediated by the formation of alternative secondary structures of the leader transcript. In vitro transcription studies with wild-type and mutant templates provided additional evidence that the identified alternative RNA secondary structures regulate transcription termination. We hypothesize that binding of a tryptophan-activated regulatory protein to a specific segment of the nascent leader transcript prevents formation of one of the alternative secondary structures, thereby directing RNA polymerase to terminate transcription.