Evidence for a composite phylogenetic origin of the plastid genome of the brown alga Pylaiella littoralis (L.) Kjellm.

The nucleotide sequence and the 5' flanking region of the rbcL gene coding for the large subunit of ribulose bisphosphate-1,5-carboxylase/oxygenase of Pylaiella littoralis, a brown alga, has been determined and the deduced amino-acid sequence has been compared to those of various photosynthetic and chemoautotrophic Eubacteria, of a red alga and of green plastids (Euglena gracilis, green algae and higher plants). Unlike the rbcL genes of green plastids which are more closely related to those of cyanobacteria, the P. littoralis rbcL gene is more closely related to that of a beta-purple bacterium, as was found for the rbcS gene of another chromophytic alga [Boczar et al., Proc Natl Acad Sci USA 86: 4996-4999, 1989]. Matrix data of homology between the rbcL gene of P. littoralis and the same gene of other organisms are presented. Based on our previous report, the gene coding for the 16S rRNA from P. littoralis is closely related to that of E. gracilis (Markowicz et al., Curr Genet 14: 599-608, 1988). We suggest that the large plastid DNA molecule of P. littoralis is a phylogenetically composite genome which probably resulted from mixed endosymbiosis events, or from a horizontal transfer of DNA.