The Serine Biosynthesis of WLY78 Is Regulated by the T-Box Riboswitch.

Serine is important for nearly all microorganisms in protein and downstream amino acids synthesis, however, the effect of serine on growth and nitrogen fixation was not completely clear in many bacteria, besides, the regulatory mode of serine remains to be fully established. In this study, we demonstrated that L-serine is essential for growth and nitrogen fixation of WLY78, but high concentrations of L-serine inhibit growth, nitrogenase activity, and expression. Then, we revealed that expression of the whose gene product catalyzes the first reaction in the serine biosynthetic pathway is regulated by the T-box riboswitch regulatory system. The 508 bp mRNA leader region upstream of the coding region contains a 280 bp T-box riboswitch. The secondary structure of the T-box riboswitch with several conserved features: three stem-loop structures, a 14-bp T-box sequence, and an intrinsic transcriptional terminator, is predicted. Mutation and the transcriptional leader- fusions experiments revealed that the specifier codon of serine is AGC (complementary to the anticodon sequence of tRNA). qRT-PCR showed that transcription of is induced by serine starvation, whereas deletion of the specifier codon resulted in nearly no expression of . Deletion of the terminator sequence or mutation of the continuous seven T following the terminator led to constitutive expression of . The data indicated that the T-box riboswitch, a noncoding RNA segment in the leader region, regulates expression of by a transcription antitermination mechanism.