In Bacillus subtilis, the adjacent genes
gltX, cysE, and cysS encoding respectively
glutamyl-tRNA synthetase, serine acetyl-transferase, and
cysteinyl-tRNA synthetase, are transcribed as an operon
but a gltX probe reveals only the presence of
a monocistronic gltX mRNA (Gagnon et al., 1994,
J Biol Chem 269:7473–7482). The transcript
of the gltX-cysE intergenic region contains putative
alternative secondary structures forming a ρ-independent
terminator or an antiterminator, and a conserved sequence
(T-box) found in the leader of most aminoacyl-tRNA synthetase
and many amino acid biosynthesis genes in B. subtilis
and in other Gram-positive eubacteria. The transcription
of these genes is initiated 45 nt upstream from the first
codon of gltX and is under the control of a σA-type
promoter. Analysis of the in vivo transcript of this operon
revealed a cleavage site immediately downstream from the
ρ-independent terminator structure. In vitro transcription
analysis, using RNA polymerases from Escherichia coli,
B. subtilis, and that encoded by the T7 phage,
in the presence of various RNase inhibitors, shows the
same cleavage. This processing generates mRNAs whose 5′-end
half-lives differ by a factor of 2 in rich medium, and
leaves putative secondary structures at the 3′ end
of the gltX transcript and at the 5′ end
of the cysE/S mRNA, which may be involved in the
stabilization of these mRNAs. By its mechanism and its
position, this cleavage differs from that of the other
known transcripts encoding aminoacyl-tRNA synthetases in
B. subtilis.