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Low molecular weight DNA in the heteromeric macronuclei of two cyrtophorid ciliates
Author(s) -
Lahlafi Touria,
Méténier Guy
Publication year - 1991
Publication title -
biology of the cell
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.543
H-Index - 85
eISSN - 1768-322X
pISSN - 0248-4900
DOI - 10.1016/0248-4900(91)90012-c
Subject(s) - biology , tetrahymena , dna , microbiology and biotechnology , gel electrophoresis , agarose gel electrophoresis , nucleic acid thermodynamics , nuclease , agarose , genetics , base sequence
Summary— The size range of the native DNA molecules in the heteromeric macronuclei of two cyrtophorid ciliates ( Trithigmostoma cucullulus, Chilodonella uncinata ) was mainly investigated by using agarose gel electrophoresis. Numerous bands superimposed on a continuous spectrum of molecular sizes between about 0.35 kb and 30 kb were resolved by conventional electrophoresis. Species‐specific banding patterns indicate a variation between species in the copy number of individual DNA fragments. A slight intra‐specific variability of banding patterns can exist. Electrophoretic distributions for two strains of T cucullulus were indeed found to differ by at least one more intense band (‘overamplified’ sequences?). Fractionation by contour‐clamped homogeneous electric field (CHEF) gel electrophoresis revealed that the size continum of macronuclear DNA molecules does not extend beyond 60–70 kb. The average size was estimated to be around 4 kb. Unresolved DNA fraction (> 1000 kb) accounted for less than 10% of the mass of cellular DNA entering CHEF gels. Macronuclear ribosomal DNA of each cyrtophorid species was identified by Southern hybridization with a Tetrahymena rDNA probe. The hybridization signal was observed on a single band of low molecular weight DNA. The corresponding size was close to 14.5 kb in Trithigmostoma and 15.5 kb in Chilodonella , which is about twice the size of monomeric rDNA in hypotrichous ciliates. We showed that S1 nuclease resistant duplexes wit half the length of the native rDNA can be formed by rapid renaturation of heat‐denatured molecules and hybridized with native rDNA. This strongly suggests that the nucleotide sequence of this rDNA is a large palindrome. Unlike the hypotrichs, macronuclear rDNA in cyrtophorids should be organized into palindromic dimers as in Tetrahymena species.