A Heme Oxygenase Isoform is Essential for Aerobic Growth in the Cyanobacterium Synechocystis sp. PCC 6803: Modes of Differential Operation of Two Isoforms/Enzymes to Adapt to Low Oxygen Environments in Cyanobacteria

Abstract

Heme oxygenase (HO) catalyzes the oxygen-dependent cleavage of heme to produce biliverdin IXα in phycobilin biosynthesis. In the genome of the cyanobacterium Synechocystis sp. PCC 6803 there are two genes, ho1 (sll1184) and ho2 (sll1875), encoding HO isoforms. Reverse transcription-PCR indicated that ho1 is constitutively expressed, and ho2 is induced under micro-oxic conditions. A mutant lacking ho1 (Δho1) failed to grow under aerobic conditions while it did grow at a significantly slower rate than the wild type under anaerobic (micro-oxic) conditions. When micro-oxically grown Δho1 was incubated under aerobic conditions, the cells underwent chlorosis with a significant decrease in phycocyanin accompanied by anomalous accumulation of protoporphyrin IX. These results suggested that HO1 is essential for aerobic growth as the sole HO and is dispensable under micro-oxic conditions. A mutant lacking ho2 (Δho2) grew under both aerobic and micro-oxic conditions like the wild type at low light intensity (50 μmol(photon) m⁻² s⁻¹). At higher light intensity (120 μmol(photon) m⁻² s⁻¹) the Δho2 mutant showed significant growth retardation under micro-oxic conditions. It is suggested that HO2 operates as a dominant HO under high light and micro-oxic environments and acts as an accessory HO at low light intensity. Constitutive expression of HO2 in a neutral site of the chromosome restored aerobic growth of Δho1, suggesting that HO2 has an activity high enough to substitute for HO1 under aerobic conditions. The differential operation of two isoforms/enzymes in cyanobacterial tetrapyrrole biosynthesis to adapt to low oxygen environments is discussed, including three other reactions.

A Novel “Oxygen-induced” Greening Process in a Cyanobacterial Mutant Lacking the Transcriptional Activator ChlR Involved in Low-oxygen Adaptation of Tetrapyrrole Biosynthesis

“…PCC 6803 and its derivative strains used in this study were cultivated in BG-11 medium as described (23 (22). DNA fragments of the upstream (f1 and r1 primers) and downstream (f2 and r2 primers) regions of the target operon were amplified by PCR from genomic DNA of Synechocystis 6803 with a standard thermal cycle (KOD-plus DNA polymerase; Toyobo, Osaka, Japan).…”

“…Synechocystis 6803 was transformed with the constructed plasmid as described (23). The mutants lacking the chlA II -ho2-hemN (sll1874-sll1875-sll1876) operon and psbA1 were named ⌬operon and ⌬psbA1, respectively.…”

“…In ⌬chlR, three genes, hemN, chlA II , and ho2, encoding oxygenindependent CPgen oxidase (HemN), low-oxygen-type MPE cyclase (ChlA II ) and heme oxygenase 2 (HO2), respectively, are not expressed under low-oxygen conditions because of the loss of chlR (24). We previously isolated three mutants, ⌬hemN, ⌬chlA II , and ⌬ho2, in which hemN, chlA II , and ho2 were disrupted individually (20,22,23). To identify which gene is responsible for the pigment accumulation in ⌬chlR, we compared the pigment profiles of these mutants with that of ⌬chlR (Fig.…”

“…MPE cyclization and heme oxygenation are catalyzed by two isoform pairs, ChlA I /ChlA II and HO1/HO2, respectively. Low-oxygen-type isoforms, ChlA II and HO2, might have some special biochemical properties enable to operate under low-oxygen conditions (22,23). The three genes, hemN, chlA II , and ho2, encoding the low-oxygen-type enzymes form a small operon, chlA II -ho2-hemN, in Synechocystis 6803, and are transcribed only under lowoxygen conditions, whereas the other three genes, hemF, chlA I , and ho1, encoding aerobic-type enzymes are expressed constitutively (20,22,23).…”

Isolation of cyanobacterial mutants exhibiting growth defects under microoxic conditions by transposon tagging mutagenesis of <i>Synechocystis</i> sp. PCC 6803

“…Chlorophyll (Chl) a is a tetrapyrrole pigment essential for photosynthesis, and the production of Chl a requires oxygen for some biosynthetic reactions. Cyanobacteria possess a gene set encoding biosynthetic enzymes that work specifically under low-oxygen conditions (Aoki et al, 2011;Fujita et al, 2015;Goto et al, 2010;Minamizaki et al, 2008). Once cells are exposed to hypoxia, the gene set for low-oxygen-type enzymes is induced to complement the production of Chl a under low-oxygen conditions.…”