Functional organization of the riboflavin biosynthesis operon from Bacillus subtilis SHgw

We have sequenced 6006 by DNA of a region from the Bacillus subtilis SHgw chromosome known to contain riboflavin biosynthesis genes (rib gene cluster, 210° on the B. subtilis genetic map). Five of the seven open reading frames found within the sequence are shown to represent the genes ribG, ribB, ribA, ribH and rib7D. The calculated molecular masses for the putative translation products are 39305, 23481, 44121, 16287 and 14574 daltons respectively. The five rib genes are transcribed as a polycistronic 4277 nucleotide messenger RNA. The steady-state level of the transcript is negatively regulated by riboflavin. A cis-acting element necessary for regulation was mapped by analysis of constitutive mutations within the 5′ untranslated region of the operon. The element is at least 48 by in length and does not bear obvious similarity to well defined prokaryotic regulatory elements. The molecular mechanism of regulation remains unknown, but the data presented argue against regulation by attenuation.