Cyanobacterial evolution: results of 16S ribosomal ribonucleic acid sequence analyses

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Abstract
The sequences of oligonucleotides released by T1-ribonuclease digestion of the 16S ribosomal RNA''s (rRNA) of the unicellular cyanobacteria Agmenellum quadruplicatum (strain BG-1) and Synechococcus 7502 are reported. They are compared with sequences previously obtained for the 16S rRNA of 6 other cyanobacteria and 2 chloroplasts, and it is concluded that: Synechocystis-like unicells form a discrete cluster which also (and surprisingly) includes Agmenellum quadruplicatum, usually considered to be a Synechococcus; filamentous cyanobacteria of the genera Nostoc and Fischerella arose from within the Synechocystis group; phylogenetic diversity (and hence presumably evolutionary antiquity) within the Synechococcus group is very great; and red algal chloroplasts are of definite cyanobacterial origin, while Euglena chloroplasts are of separate and quite possibly noncyanobacterial origin. We also present the results of a computer-aided search among the 10 oligonucleotide ''catalogues'' for families of related but nonidentical sequences. Examination of these families reinforces the above conclusions.