The 5S ribosomal RNA genes of the oomycete Pythium irregulare exist in
tandem arrays unlinked to the rDNA repeat unit. A clone with a 9.2-kb
insert containing an array of 5S genes was identified in a lambda gen
omic library and was characterized by restriction mapping and partial
sequencing. The array consisted of 9 apparently identical 5S genes and
their spacers in tandem, followed by a diverged 5S-like sequence that
is likely to be a pseudogene. This gene arrangement, although almost
universal in plants and animals, is rare in fungi and protists.