THE TRP RNA-BINDING ATTENUATION PROTEIN (TRAP) REGULATES THE STEADY-STATE LEVELS OF TRANSCRIPTS OF THE BACILLUS-SUBTILIS FOLATE OPERON

The Bacillus subtilis folate operon contains nine genes. The first six genes are involved in the biosynthesis of folic acid and tryptophan a nd have been characterized previously. The 3'-region of the folate ope ron contains three additional ORFs: orf3, potentially encoding a DNA-b inding protein of 68 amino acids, orf4, encoding a protein of 338 amin o acids with homology to the Orf1 of the E. coli fis operon, and a put ative lysyl-tRNA synthetase gene (lysS). Four transcripts were identif ied which encode the first two, eight or all nine proteins or only the last protein LysS. The folate operon contains two promoters, one upst ream of the first gene and the second preceding lysS. Transcription of the entire folate operon starts 33 bp upstream of the ATG codon of pa b, the first gene of the operon. The mtrB-encoded trp RNA-binding atte nuation protein (TRAP) dramatically reduces the steady-state levels of the folate operon transcripts encoding the first eight and all nine p roteins, but only has a relatively small effect on the steady-state le vel of the 2.1 kb transcript encoding the first two genes of the opero n, pab and trpG. In addition, transcription of the folate operon is re gulated in a growth-phase-dependent manner. Transcripts were present i n very low levels after mid-exponential phase, but were dramatically i ncreased directly after transfer of the cells to fresh medium. These r esults indicate that transcription of the folate operon is regulated b y TRAP and also depends on the growth phase of the culture.