Chloroplasts as the major protein-bearing structures in Oenothera leaves

Abstract

Analyses of chloroplasts isolated from mature leaves of Oenothera (evening primrose) have shown that plastids contain 75% of the leaf protein. Chloroplasts are thus the major source of proteins in these leaves. It is suggested that plastids are also the major sites of protein synthesis in the leaf. Preparation of protein-rich chloroplasts was possible because the plastids lost little protein if isolated in a sucrose medium containing phosphate as the buffering ion. However, if borate was substituted for phosphate, Oenothera chloroplasts lost almost half of their proteins during isolation. The use of borate in the isolating medium rather than phosphate also reduced severely the ability of chloroplasts to fix CO 2 in light. Dilute phosphate buffer, a poor extractant of proteins from isolated chloroplasts, extracted little protein from mature Oenothera leaves, whereas borate, an efficient extractant of chloroplast proteins, was also a good extractant of protein from the whole leaf. Furthermore, most soluble proteins obtained from leaves by extraction with borate were shown both quantitatively and by electrophoretic comparison to be chloroplast proteins. Therefore, many soluble proteins of Oenothera leaves are not cytoplasmic proteins, but originate mainly in chloroplasts.