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XVI.A.1.b. PSI-B
Sequences of PSI-B
Sequence Similarity Search for PSI-B
XVI.A.1.b. PSI-B
PSI-A and PSI-B probably in the form of a heterodimer constitute the P700-Chl a protein I, also known as CPI. CPI consists of the primary reaction center P700, the primary electron acceptors A0 and A1, and the iron -sulfur center X. The two polypeptides bind about 90 molecules of Chl a, and 10-15 molecules of carotene per P700 (for a review, see Andersen and Scheller, 1993).
PSI-B is encoded by the psaB gene, which is located on the chloroplast genome as a single copy. The psaA gene encode a polypeptide of 733-751 amino acids. The polypeptide is highly conserved among all analyzed species. It exhibits a homology of about 45 % to psbA, suggesting that the two genes are the result of gene duplication.
The PSI-A polpypeptide is hydrophobic. Hydropathy plots which can be achieved via the Biology Workbench, suggest the presence of 11 membrane-spanning alpha-helixes. The hydrophobic segments are rich in histidine residues which are implicated in Chl binding.
The amount of acid-labile sulfur and nonheme iron bound to PSI-A and PSI-B is about 4 mol of acid labile sulfur and 4 mol pf nonheme iron per mole of P700. This amount can in turn accommodate either one [4Fe-4S] or two [2Fe-2S] clusters. Binding of both types of clusters require four cysteine residues. PSI-A, contains only three cysteine residues, while PSI-B contains only two. Since only two 80-kDa subunits (corresponding to PS1-A and PSI-B) are present per P700, it was concluded that only a single iron-sulfur cluster could be bound, which leads to the conclusion that center X is probably [4Fe-4S]. Center X is presumed to be bridged between PSI-A and PSI-B with each polypeptide subunit contributing two cysteine residues.
In the vicinity of the conserved cysteine residues, on one side of the alpha-helix, every seventh amino acid is a leucine residue. it is believed that the leucine residues act as a zipper, that ensures the appropriate conformation of the polypeptides for the proper insertion and stabilization of the iron-sulfur cluster.
A query for psaB addressed to the Various protein databases listed in the Biology Workbench, yielded 46 records which are depicted under Sequenced PSI-B. These sequences can be used for Sequence Similarity Searches or other manipulations using one of several routines described in the Biology Workbench. For example a Blast Sequence Similarity Search of spinach PSI-B addressed to the Biology Workbench (SwissProt database) is depicted below.
Sequences of PSI-B
Click on Sequences to view the sequences of PSI-B retrieved from various protein sequence databases by the Biology Workbench.
Sequence Similarity Search for PSI-B
Click on Sequence Similarity Search to view the Blast sequence similarity search results of spinach PSI-B from the Biology workbench (SwissProt database).
References
- Andersen, B. and H. V. Scheller (1993). Structure, Function and Assembly of photosystem I. In: Pigment-Protein Complexes In Plastids: Synthesis and Assembly . C. Sundqvist and M. Ryberg, (eds.), p 383-417, Academic Press, New York.