Molecular Analysis of Ribosomal RNA Genes in Rhizoctonia Fungi

Abstract

The DNA sequences that encode ribosomal RNAs have been useful for understanding phylogenetic and taxonomic relationships and patterns of genetic variation in fungi (Bruns et al.,1992). In fungi, ribosomal genes (rDNA) are located within mitochondria or nuclei and contain numerous regions of high sequence conservation and variability over their entire length (White et al., 1990). The nuclear rDNA genes of fungi are arranged as a series of tandemly repeated units that include genes for the 18S (small), 5.8S and 28S (large) rRNA subunits. Vilgalys and Gonzalez (1990a) determined that Thanatephorus praticola (anamorph = Rhizoctonia solani AG 4) had approximately 59 copies (repeated units) per haploid genome. The 28S and 18S rRNA subunits are separated by an intergenic spacer (IGS) region that usually contains a 5S rRNA subunit and the 5.8S subunit is flanked by two internal transcribed spacer (ITS 1 and ITS2) regions. In contrast to nuclear rDNA genes, mitochondrial (mt) rDNA genes have a lower guanine and cytosine content (25–40%), possess more methylated nucleotides and lack a 5.8S subunit (White et al.,1990). Except for analysis of the small mt rRNA subunit (Liu and Sinclair, 1992), mt rDNA genes have not been critically examined in Rhizoctonia fungi.