Functional and structural differences between photosynthetic and heterotrophic Rhodospirillum rubrum ribosomes and S-100 fractions.

Cell-free, protein-synthesizing activity has been tested by using various combinations of the S-100 and ribosome fractions prepared from photosynthetic and heterotrophic Rhodospirillum rubrum. The photosynthetic ribosomes are highly active when combined with either the photosynthetic or the heterotrophic S-100 fractions, whereas the heterotrophic ribosomes are active only when combined with the photosynthetic S-100 fraction. Addition of a photosynthetic pigment-containing fraction to the homologous heterotrophic system is, however, able to stimulate its activity. An inhibitor and an activator involved in cell-free protein synthesis have been isolated from the stationary heterotrophic cells. The inhibitor is a very small, dialyzable compound which inhibits not only the R. rubrum but also the E. coli protein-synthesizing activity in vitro, whereas the activator is a non-dialyzable, small RNA molecule capable of stimulating only the R. rubrum activity. Differences exist between the photosynthetic and the heterotrophic systems in their response to various chemical compounds and to light as well as in their structure.