FIG. 1.

Length of the ITS region between the 16S and 23S rDNAs in Prochlorococcus and Synechococcus isolates. Low-B/A Prochlorococcus isolates all have short spacers, while high-B/A isolates have longer spacer lengths and are not as similar to one another. The freshwater strain PCC 6307 has a much longer spacer than the marine Synechococcus and Prochlorococcus isolates. The G+C content of the 16S-23S rDNA ITS is indicated to the right of each bar. Low-B/A Prochlorococcus and four high-B/A Prochlorococcus isolates have a lower G+C content than two high-B/A Prochlorococcus and Synechococcus isolates. For sets of isolates with identical ITS sequences (see text), only one isolate is represented here.