PsaE is a small basic subunit located on the stromal (cytoplasmic) side of photosystem I. In cyanobacteria, this subunit participates in cyclic electron transport and modulates the interactions of the complex with soluble ferredoxin. The PsaE protein isolated from the cyanobacterium Synechococcus sp. strain PCC 7002 adopts the beta topology of an SH3 domain, with five beta strands (betaA through betaE) and a turn of 3(10) helix between strands betaD and betaE [Falzone, C. J., Kao, Y.-H., Zhao, J., Bryant, D. A., and Lecomte, J. T. J. (1994) Biochemistry 33, 6052-6062]. The primary structure of the PsaE protein is strongly conserved across all oxygen-evolving photosynthetic organisms. However, variability in loop lengths, as well as N- or C-terminal extensions, suggests that the structure of a second representative PsaE subunit would be useful to characterize the interactions among photosystem I polypeptides. In this work, the solution structure of PsaE from the cyanobacterium Nostoc sp. strain PCC 8009 was determined by NMR methods. Compared to PsaE from Synechococcus sp. strain PCC 7002, this PsaE has a seven-residue deletion in the loop connecting strands betaC and betaD, and an eight-residue C-terminal extension. Angular and distance restraints derived from homonuclear and heteronuclear NMR experiments were used to calculate structures by a distance-geometry/simulated-annealing protocol. A family of 20 structures (rmsd of 0.24 A in the regular secondary structure) is presented. Differences between the two cyanobacterial proteins are mostly confined to the CD loop region; the C-terminal extension is disordered. The thermodynamic stability of Nostoc sp. strain PCC 8009 PsaE toward urea denaturation was measured by circular dichroism and fluorescence spectroscopy, and thermal denaturation was monitored by UV absorption spectroscopy. Chemical and thermal denaturation curves are modeled satisfactorily with two-state processes. The DeltaG degrees of unfolding at room temperature is 12.4 +/- 0.3 kJ mol(-1) (pH 5), and the thermal transition midpoint is 59 +/- 1 degrees C (pH 7). Interactions with other proteins in the photosystem I complex may aid in maintaining PsaE in its native state under physiological conditions.