Effects of Modified Phycobilin Biosynthesis in the CyanobacteriumSynechococcussp. Strain PCC 7002

Abstract

ABSTRACTThe pathway for phycocyanobilin biosynthesis inSynechococcussp. strain PCC 7002 comprises two enzymes: heme oxygenase and phycocyanobilin synthase (PcyA). The phycobilin content of cells can be modified by overexpressing genes encoding alternative enzymes for biliverdin reduction. Overexpression of thepebABand HY2 genes, encoding alternative ferredoxin-dependent biliverdin reductases, caused unique effects due to the overproduction of phycoerythrobilin and phytochromobilin, respectively. Colonies overexpressingpebABbecame reddish brown and visually resembled strains that naturally produce phycoerythrin. This was almost exclusively due to the replacement of phycocyanobilin by phycoerythrobilin on the phycocyanin α-subunit. This phenotype was unstable, and such strains rapidly reverted to the wild-type appearance, presumably due to strong selective pressure to inactivatepebABexpression. Overproduction of phytochromobilin, synthesized by theArabidopsis thalianaHY2 product, was tolerated much better. Cells overexpressing HY2 were only slightly less pigmented and blue-green than the wild type. Although thepcyAgene could not be inactivated in the wild type,pcyAwas easily inactivated when cells expressed HY2. These results indicate that phytochromobilin can functionally substitute for phycocyanobilin inSynechococcussp. strain PCC 7002. Although functional phycobilisomes were assembled in this strain, the overall phycobiliprotein content of cells was lower, the efficiency of energy transfer by these phycobilisomes was lower than for wild-type phycobilisomes, and the absorption cross-section of the cells was reduced relative to that of the wild type because of an increased spectral overlap of the modified phycobiliproteins with chlorophylla. As a result, the strain producing phycobiliproteins carrying phytochromobilin grew much more slowly at low light intensity.