Abstract
Heme oxygenase (HO) catalyzes the oxygen-dependent cleavage of the porphyrin ring of heme producing biliverdin IXα in the phycobilin biosynthesis. There are two genes, ho1 (sll1184) and ho2 (sll1875), encoding heme oxygenase isoforms in the cyanobacterium Synechocystis sp. PCC 6803. Previous study revealed that a mutant lacking ho1 (Δho1) does not grow under aerobic conditions suggesting that ho1 is essential for growth under aerobic conditions. In this study, we isolated a pseudo-revertant R1 from Δho1 and characterized it. R1 grew slightly slower than wild type under aerobic conditions. The phycocyanin content was almost comparable to that of wild type and the anomalous accumulation of protoporphyrin IX in Δho1 was significantly relieved. RT-PCR indicated that ho2 is expressed in a constitutive manner in R1 while ho2 is repressed under aerobic conditions and induced under low-oxygen conditions in wild type. Thus, it is suggested that the aberrant constitutive expression of ho2 restores the defect in ho1 resulting in the growth recovery of Δho1 under aerobic conditions. To identify the relevant mutation sites, genome analysis of R1 is under progress.
