Disruption of a gene encoding a novel thioredoxin-like protein alters the cyanobacterial photosynthetic apparatus

Abstract

A gene that may encode a novel protein disulfide oxidoreductase, designated txlA (thioredoxin-like), was isolated from the cyanobacterium Synechococcus sp. strain PCC7942. Interruption of txlA near the putative thioredoxin-like active site yielded cells that grew too poorly to be analyzed. In contrast, a disruption of txlA near the C terminus that left the thioredoxin-like domain intact yielded two different mutant phenotypes. One type, designated txlXb, exhibited a slightly reduced growth rate and an increased cellular content of apparently normal phycobilisomes. The cellular content of phycobilisomes also increased in the other mutant strain, designated txlXg. However, txlXg also exhibited a proportionate increase in chlorophyll and other components of the photosynthetic apparatus and grew as fast as wild-type cells. Both the txlXb and txlXg phenotypes were stable. The differences between the two strains may result from a genetic polymorphism extant in the original cell population. Further investigation of txlA may provide new insights into mechanisms that regulate the structure and function of the cyanobacterial photosynthetic apparatus.The conversion of light energy into chemical energy (ATP) and reducing power (NADPH) by oxygen-evolving photosynthetic organisms requires the coordinated activity of photosystem I (PSI) and photosystem II (PSII), along with their lightharvesting antenna pigments and intermediary electron transport components. NADPH is produced only by linear electron flow through both PSII and PSI, but ATP can be produced either by linear electron flow or by cyclic electron flow around PSI alone (24, 34). In cyanobacteria, PSI is excited almost entirely by light absorbed by chlorophyll a (Chl; A max , 440 and 680 nm), while PSII receives most of its excitation energy from light absorbed by the phycobilisomes (PBS; A max , 560 to 620 nm [32]). Within these basic functional constraints, the structure of the photosynthetic apparatus is dynamic. For example, while the PBS-to-Chl ratio generally reflects the PSII-to-PSI ratio, under some conditions cyanobacteria can modulate the light-harvesting capacity of PBS relative to Chl independently of the PSII-to-PSI ratio by changing the size of the PBS (11,20,40) or the number of PBS relative to PSII (42). Cyanobacteria grown in light that is harvested primarily by PBS show a decline in the PSII-to-PSI ratio, while those grown in light harvested primarily by PSI show the opposite response (1,26,41,44). These changes may allow cyanobacteria to balance the production of ATP and NADPH despite conditions favoring the activity of one photosystem over the other (26,43). Carbon-limited cyanobacteria show a decline in the ratio of PSII to PSI that may reflect an increase in cyclic relative to linear photosynthetic electron flow. This change could increase the production of ATP relative to NADPH and help meet the extra demand for energy incurred by cells that must actively take up inorganic carbon (36,45,51,53). Patterns of electron transport are also altered whe...

HCF164 Encodes a Thioredoxin-Like Protein Involved in the Biogenesis of the Cytochrome b 6 f Complex in Arabidopsis

“…The function of the TxlA proteins is not yet clear. Partial gene knockouts yielded a severe defect in photoautotrophic growth (Collier and Grossman, 1995). However, the results remained inconclusive because the cells grew poorly, and therefore detailed biochemical analyses could not be performed.…”

Thioredoxins: structure and function in plant cells

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HCF164 Encodes a Thioredoxin-Like Protein Involved in the Biogenesis of the Cytochrome b6f Complex in Arabidopsis

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