Photosystem I, an improved model of the stromal subunits PsaC, PsaD, and PsaE

An improved electron density map of photosystem I (PSI) calculated at 4-Ang strom resolution yields a more detailed structural model of the stromal sub units PsaC, PsaD, and PsaE than previously reported. The NMR structure of t he subunit PsaE of PSI from Synechococcus sp, PCC7002 (Falzone, C, J., Kao, Y.-H., Zhao, J., Bryant, D. A, and Lecomte, J, T, J, (1994) Biochemistry 3 3, 6052-6062) has been used as a model to interpret the region of the elect ron density map corresponding to this subunit, The spatial orientation with respect to other subunits is described as well as the possible interaction s between the stromal subunits. A first model of PsaD consisting of a four- stranded beta-sheet and an alpha-helix is suggested, indicating that this s ubunit partly shields PsaC from the stromal side. In addition to the improv ements on the stromal subunits, the structural model of the membrane-integr al region of PSI is also extended. The current electron density map allows the identification of the N and C termini of the subunits PsaA and PsaB, Th e 11-transmembrane alpha-helices of these subunits can now be assigned uniq uely to the hydrophobic segments identified by hydrophobicity analyses.