Antisense RNA asPcrL regulates expression of photosynthesis genes in by promoting RNase III-dependent turn-over of mRNA.

Anoxygenic photosynthesis is an important pathway for to produce ATP under oxygen-limiting conditions. The expression of its photosynthesis genes is tightly regulated at transcriptional and post-transcriptional levels in response to light and oxygen signals, to avoid photooxidative stress by the simultaneous presence of pigments, light and oxygen. The operon encodes pigment-binding proteins of the light-harvesting complex I (genes and ), of the reaction centre (genes and ), a scaffold protein (gene ) and includes the gene for sRNA PcrX. Segmental differences in the stability of the mRNA contribute to the stoichiometry of LHI to RC complexes. With asPcrL we identified the third sRNA and the first antisense RNA that is involved in balancing photosynthesis gene expression in . asPcrL influences the stability of the mRNA but not of the mRNA and consequently the stoichiometry of photosynthetic complexes. By base pairing to the region asPcrL promotes RNase III-dependent degradation of the mRNA. Since asPcrL is activated by the same protein regulators as the operon including PcrX it is part of an incoherent feed-forward loop that fine-tunes photosynthesis gene expression.[Figure: see text].