The chlorophyll a/b‐protein complex (LHCPI) associated with Photosystem I (PS I) has been isolated from spinach thylakoids and further fractionated into two chlorophyll‐containing complexes by sucrose gradient centrifugation. The lighter fraction contains two polypeptides with relative molecular masses of 23 and 22 kDa and has been designated as LHCPIa. The denser fraction is enriched in a 20 kDa polypeptide and has been named LHCPIb. Both fractions have a chlorophyll a/b ratio of 3.5 ± 0.5. The absorption spectra and 77 K fluorescence emission spectra of the fractions show distinct characteristics with LHCPIb having a fluorescence maximum at 730 nm at 77 K while LHCPIa shows a maximum at 680 nm. The optical activities of the chlorophyll a/b complexes and the antenna‐depleted PS I have been examined by circular dichroism (CD) in the near‐UV and visible regions of the spectrum. All the LHCPI complexes show strong CD signals at 648, 485 and 340 nm which are absent in the antenna‐depleted PS I complex.

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Chloroplasts can be obtained by gentle lysis or mild shear of spheroplasts of vitamin B12-deficient Euglena gracilis and then purified by isopycnic sedimentation on gradients of Ludox AM or Percoll. The chloroplasts appear compact and highly refractile by phase contrast or Hoffmann contrast microscopy. Upon incubation with 13HIleucine or 135Slmethionine, the chloroplasts incorporate the amino acids into protein at rates that are 100-fold faster than we had previously observed with Euglena and up to 8-fold faster than with chloroplasts of spinach. Eugkna chloroplasts prepared by the current procedure are thus qualitatively superior to those previously available from Eugkna and at least as active in protein synthesis as chloroplasts from higher plants.Although Euglena gracilis offers many advantages in the study of chloroplast development, a serious limiting factor in pursuing the molecular biology of this process has been the difficulty in isolating pure, intact, and functional plastids from Euglena. The pioneer studies of Eisenstadt and Brawerman (5) were based on crude fractions obtained by differential centrifugation and differential flotation in sucrose. Although the plastids obtained were extensively contaminated with other subcellular particles and the integrity of the organelles was never assessed, their isolation procedures were nonetheless followed by a number of other laboratories. We now realize that the high osmotic pressures required for flotation in sucrose gradients are incompatible with the integrity of chloroplasts.Several investigators have used spheroplasts in an effort to isolate plastids under relatively mild conditions (3,8,11,13), but the chloroplasts obtained lacked stroma and the envelopes were broken or lost.We have described (18) a separation procedure in which the chloroplasts were separated by rate zonal centrifugation in isosmotic gradients of Ficoll. This method yielded chloroplasts that appeared intact by electron microscopy but were not functional. We then found (10) that pure, intact, and functional chloroplasts

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