snRNP-like proteins associated with the thylakoid membranes and thylakoid/phycobilisomes fractions of the cyanobacterium, Synechococcus leopoliensis

Small ribonucleoprotein particles (snRNPs) are involved in the splicing of pre-messenger RNA and were predominantly thought to be present in eukaryotic systems only. Recent studies have indicated the presence of snRNP-like components in a prokaryotic cyanobacterium, Synechococcus leopoliensis. To determine the cellular location of the snRNP-like material within it, the membranes of the cyanobacterium were fractionated by flotation centrifugation on a sucrose density gradient. Proteins associated with the plasma, thylakoid membranes, and thylakoid/phycobilisome fractions were immunoprecipitated using Sm and RNP antisera, which are specific for snRNP antigens. Both the thylakoid membranes and thylakoid/phycobilisomes fractions were found to contain differentially distributed snRNP-reactive proteins comparable in size to the snRNP polypeptides immunoprecipitated from the nuclear fraction of human cells. As chloroplasts of higher plants are considered to be descendents of the cyanobacteria, the presence of snRNP-like components in cyanobacteria offers an additional link for the evolution of snRNP-mediated mRNA splicing.