Thylakoid formation1 (Thf1), encoded by sll1414 (thf1), is a multifunctional
protein conserved in all photosynthetic organisms. thf1 expression is highly induced by high light in Synechocystis during photosynthesis-related stress. In this study, differential
RNA sequencing analysis of the Synechocystis sp. PCC 6803 revealed a small
antisense RNA (asRNA) gene located on the reverse-complementary strand of the thf1 gene. The full length of this asRNA (designated ThfR) was determined by 5' and 3' RACE analysis. The accumulation of thf1
mRNA was up-regulated synchronously with the ThfR level during survival after high-light stress or
nitrogen starvation. Under
nitrogen starvation or high-light stress, compared with the wild type, a ThfR overexpression mutant demonstrated relatively more Thf1
protein content, while a ThfR reduced-expression mutant accumulated less Thf1
protein. Furthermore, the overexpression of ThfR enhanced the electron transport rate and the proliferation of cyanobacteria under high-light stress. These results, which we confirmed further using an Escherichia coli sRNA expression platform, suggest that the thf1 gene is positively regulated by ThfR, possibly through protection of the RAUUW
element at the
RNase E cleavage site. This study represents the first report, to our knowledge, of a cis-transcript
antisense RNA that targets thf1 in Synechocystis sp. PCC 6803 and provides evidence that ThfR regulates photosynthesis by positively modulating thf1 under high-light conditions.