Abstract: Synechocystis sp.PCC 6803 is suitable for studies of photosynthesis.Further,slr1122 encodes an unknown protein of 250aa,which possibly interacts with a hybrid sensory kinase encoded by sll1672 (Hik12).Via pull-down assay, we verified that Slr1122 interacted with Sll1672.With the isotope assay of ³²P,the auto-phosphorylation of Hik12 decreased in the presence of Slr1122,indicating that Slr1122 regulated the two-component system.By homologous double-crossover,slr1122 was replaced with antibiotic resistance cassettes kanamycin,resulting in the knockout mutant of Synechocystis sp.PCC 6803,Δslr1122.We found that in Δslr1122,the transcription of the key PSⅡ component gene,psbAI (slr1181),decreased remarkably,leading to low capacity of photosynthesis and low growth rate of Δslr1122 compared with the wild type.In addition,Δslr1122 became more sensitive to light.Under low light,Δslr1122 caught more light and grew more rapidly,with the opposite occurring under high light,and thus Δslr1122 was more liable to photodamage.Δslr1122 had lower content of phycobiliprotein and photosynthetic pigments,particularly carotenoids.RT-PCR showed that the five important genes involved in carotenoid biosynthesis were transcribed more lowly.Coincident with this,Δslr1122 was more sensitive to redox stress and high light stress.In conclusion,Slr1122 was involved in the regulation of phosphorylation of Hik12 and thereby influenced the biosynthesis of carotenoids.