11.2.4.1—
Ribosomes

Lyttleton (1962) was the first to show that green plant cells contain two classes of ribosome which differ in their sedimentation coefficients. Chloroplasts contain 70s ribosomes while the cytoplasm contains 80s ribosomes. Figure 11.5 shows a density gradient analysis of an homogenate of spinach leaves, from which it can be seen that chloroplast ribosomes constitute a high proportion of the total cellular complement of ribosomes; values as high as 60% have been reported.

Chloroplast ribosomes resemble those from prokaryote cells in their s value. Their RNA components are also of the same size as those found in prokaryote ribosomes, i.e. 16s and 23s. The 80s cytoplasmic ribosomes by contrast, contain 18s and 25s RNA molecules. These similarities between chloroplast and prokaryote ribosomes have been much stressed in the past, but it is now clear that these similarities are of size only, and not in the primary structure of the RNA and protein components. For example, ribosomal-RNA from Escherichia coli does not compete with chloroplast ribosomal-RNA from Euglena for hybridization to chloroplast DNA. The protein complement of prokaryote ribosomes is also quite different from that of chloroplast ribosomes as judged by gel electrophoresis and immunological tests. Similarly, the proteins of chloroplast and cytoplasmic ribosomes from the same plant differ distinctly in the patterns they give on gel electrophoresis.

Some of the chloroplast ribosomes are bound to the internal membranes. Electron microscopy has revealed whorl-like polyribosomes attached to the granal and intergranal thylakoid membranes, while analysis of isolated chloroplasts shows that up to 50% of the ribosomes cannot be removed from the membranes by washing with hypotonic buffer. Tao and Jagendorf (1973) have

Figure 11.5
Proportion of chloroplast ribosomes in leaves of  Spinacia
oleracea.  Extracts of spinach leaves were treated with
detergent to release membrane-bound ribosomes, and all the
ribosomes collected by centrifugation. The ribosomes
were analysed on a linear sucrose density gradient.
(Unpublished work of M. R. Hartley.)

shown that some free ribosomes are lost during chloroplast isolation without the chloroplasts lysing irreversibly; when this loss is taken into account, they estimate that in chloroplasts from Pisum sativum, about 20% of the ribosomes are membrane-bound while the remainder are free in the stroma. There is evidence that the two classes of chloroplast ribosome synthesize different types of protein (see 11.2.4.6).