Abstract
Photosystem I is a multisubunit pigment-protein complex that functions as a light-driven plastocyanin-ferredoxin oxidoreductase in thylakoid membranes of cyanobacteria and higher plants. A 16-kDa protein subunit of photosystem I complex was isolated from the cyanobacterium Synechocystis sp. PCC 6803. The sequence of its NH2-terminal residues was determined and a corresponding oligonucleotide probe was used to isolate the gene encoding this subunit. The gene, designated as psaL, codes for a protein of 16,605 Da. The deduced amino acid sequence is homologous to the subunit PsaL of barley photosystem I. There are two conserved hydrophobic regions in the subunit PsaL that may cross or interact with thylakoid membranes. The gene psaL exists as a single copy in the genome and is expressed as a monocistronic RNA. Stable mutant strains in which the gene psaL was interrupted by a gene conferring resistance to chloramphenicol, were generated by targeted mutagenesis. Growth and photosynthetic characteristics of a selected mutant strain under photoautotrophic conditions were similar to those of the wild type, suggesting that the function of PsaL is dispensable for photosynthesis in Synechocystis sp. PCC 6803. Western analysis and subunit composition of purified photosystem I revealed that the mutant strain contained other subunits of photosystem I in thylakoid membranes and in the assembled complex. When photosystem II activity was inhibited and glucose was supplied in the medium, mutant strains grew faster than the wild type. Under these conditions of growth, re-reduction of P700 in the mutant cells, but not in the wild type cells, showed a component with an uncharacteristically rapid half-time.
Highlights
Introduction of anadditionalEcoRI site in the mutant chro- FIG.6
Both strands of a 700-bp Sau3A fragment containing the psaL peptide was isolated from Photosystem I (PS I) as described and an NHZ
Thereforethe type and ALC7-3 strains were pelleted and strain ALC7-3 contains only the interrupted Growth of the mutant andwild type strains psagLene. was monit ore d thenresuspendedin supplemented with 5 mM glucose and
Summary
Growth of wild typeand ALC7-3 strainsof Synechmosome wasexpectedsincethegeneforchloramphenicol ocystis sp.PCC 8803. R G l l medium (triangles),in B G l l medium 5 mM glucose (circles), or in B C l l medium with 10 p~ DCMU (diamonds). The gmwth of these by measuring absorbance of cultures at 730 nm (Fig. 6). The cultures was monitored by measuring their absorbance a t 730 nm a t photoautotrophic growth rate of ALC7-3 in B G l l was not different time intervals. Each treatment was replicated three times significantlydifferentfrom that of wild typecells (Fig. 6, to determine averages and standard deviatio(nnhown by bars)
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