VARIANT FORMS OF A GROUP-I INTRON IN NUCLEAR SMALL-SUBUNIT RIBOSOMAL-RNA GENES OF THE MARINE RED ALGA PORPHYRA-SPIRALIS VAR AMPLIFOLIA

A group IC1 intron occurs in nuclear small-subunit (18S) ribosomal RNA (SSU rRNA) genes of the marine red alga Porphyra spiralis var. amplif olia. This intron occurs at the same position as the self-splicing gro up IC1 introns in nuclear SSU rDNAs of the fungus Pneumocystis carinii and in the green alga Chlorella ellipsoidea and shares sequence ident ity with the Pneumocystis carinii intron in domains L1, P1, P2, and L2 , outside the conserved core. Three size variants, differing in amount of sequence in L1, exist and are differentially distributed in geogra phically distinct populations. Preliminary data suggest that the large st variant can self-splice in vitro. Short open reading frames are pre sent but do not correspond to known genes. Repeated nucleotide motifs, reminiscent of duplicated target sites of transposons or Alu elements , are associated with the intron and with one of the variant forms of L1. Insertions are present in nuclear SSU rDNAs of several other Porph yra species and of the red alga Bangia atro-pulpurea, insertionless rD NA variants also occur in several Porphyra species. Our observations a re most readily explained by intron mobility, although it remains uncl ear how transfer could have been mediated between genomes of organisms as ecologically diverse as marine red algae, freshwater green algae, and a mammalian-pathogenic fungus.