Antisense RNA regulates glutamine synthetase in a heterocyst-forming cyanobacterium

Glutamine synthetase (GS) is a key enzyme involved in nitrogen assimilation and the maintenance of C/N balance, and it is strictly regulated in all bacteria. In cyanobacteria, GS expression is controlled by nitrogen control A (NtcA) transcription factor, which operates global nitrogen regulation in these photosynthetic organisms. Furthermore, posttranslational regulation of GS is operated by protein-protein interaction with GS inactivating factors (IFs). In this study, we describe an additional regulatory mechanism involving an antisense RNA. In Nostoc sp. PCC 7120, the gifA gene (encoding GS inactivating factor IF7) is transcribed downstream of the GS (glnA) gene, from the opposite strand, and the gifA mRNA extends into the glnA coding sequence in antisense orientation. Therefore, the dual RNA transcript that encodes gifA constitutes two functional regions: a 5' protein-coding region, encoding IF7, and a 3' untranslated region that acts as an antisense to glnA. By increasing the levels of such antisense RNA either in cis or in trans, we demonstrate that the amount of GS activity can be modulated by the presence of the antisense RNA. The tail-to-tail disposition of the glnA and gifA genes observed in many cyanobacterial strains from the Nostocales clade suggests the prevalence of such antisense RNA-mediated regulation of GS in this group of cyanobacteria.