The Proteins of Green Leaves

Previous work (1) has shown that about 50 per cent of the total protein nitrogen contained in a tobacco leaf which had recently completed growth by expansion is associated with particulate structures of leaf protoplasm such as nuclei, chloroplasts, etc. The other 50 per cent of the protein nitrogen remains soluble in the presence of neutral buffers. In the analytical ultracentrifuge, the soluble proteins (cytoplasmic proteins) are resolved into two distinct components: (1) Fraction I which is homogeneous by the criteria of centrifugation and electrophoresis and (2) Fraction II which is a heterogeneous mixture of proteins. Fraction I possesses a sedimentation constant of about 19 S, while Fraction II has a mean sedimentation constant of about 4 S. The purpose of this investigation has been to extend previous observations which were confined to the cytoplasmic protein composition of leaves at one stage of development. This communication presents analytical data on the synthesis and decay of Fraction I and Fraction II proteins during most of the life span of attached leaves of two tobacco varieties. The picture that emerges is that of a highly dynamic condition of leaf protoplasm in which both the ratio of Fraction I to Fraction II and their amounts constantly change with time. However, the study also reveals that t,hesc changes follow an orderly and predictable course that can be directly correlated with the stage of development of the leaf.