Fluorescence emission spectra at 4 K of the D1-D2-Cytb559 reaction center complex of photosystem II, presumably containing six chlorophyll a (Chl a) and two pheophytin a (Pheo a) molecules, were obtained by selective excitation with a tunable, continuous laser ([lambda][sub exc] = 680-691 mm). The bands in the vibrational region (700-800 nm) showed a characteristic structure, which could be used as a [open quotes]fingerprint[close quotes] to identify the emitting pigment at low temperature. By comparison of this [open quotes]fingerprint[close quotes] with those of monomeric Chl a and Pheo a in Triton X-100 detergent, it is unambiguously shown that the steady-state emission arises from a Chl a species for [lambda][sub exc] > 683 nm. The nonselectively excited emission spectrum ([lambda][sub exc] < 679 nm) reveals primarily Chl a emission bands, peaking at 683.7 [+-] 0.3 and 744.0 [+-] 0.5 nm. The shape of this spectrum is significantly different from that of earlier reported emission spectra of D1-D2-Cytb559 complexes with presumably a lower Chl a content (four to five Chl/2 Pheo). These show characteristic bands for Pheo a emission. We conclude that D1-D2-Cytb559 complexes with a higher Chl a content (6 Chl/2 Pheo) contain an additional long-wavelength (680-683 nm) Chl a molecule,more » which is not related to P-680. The steady-state fluorescence at low temperatures mainly arises from emission of this Chl and not from charge-recombination fluorescence of P-680. 28 refs., 5 figs., 1 tab.« less