Specific organization of ribosomal DNA arrays in Squamata
The organization of ribosomal genes in the genome and amplified nucleoli of loach (Misgurnus fossilis L.), a teleost fish, was studied. Ribosomal DNA (rDNA) was isolated by density gradient centrifugation, digested by EcoRI endonuclease and mapped with 18S and 28S rRNAs. The transcription unit of ribosomal genes in oocyte nucleoli was investigated by an electron-microscopic visualization technique. The repeat unit of loach rDNA was shown to be split by EcoRI into two fragments: a constant fragment (3 · 106 daltons) and heterogeneous ones (11–13 · 106 daltons, major type, and 7–8 · 106 daltons, minor type). The average size of the transcription units was found to be 2.5±0.2 μm. The lengths of nontranscribed spacers varied from 2.4 to 2.9 μm; however, shorter ones of 1.2–1.4 μm were also noticed. The data indicate that loach rDNA is organized as tandem repeat units of two types: a major fraction of 20–25 kb (14–17 · 106 daltons) and a minor one 15–16 kb (10–11 · 106 daltons). The heterogeneous length of the repeat units was predetermined by the heterogeneity of nontranscribed spacers.