The Genes for Cytoplasmic Ribosomal Ribonucleic Acid in Higher Plants

Abstract

The genes for cytoplasmic ribosomal RNA are partially resolved from the bulk of the DNA by CsCI equilibrium centrifugation. Although in some plants the buoyant density of the ribosomal RNA genes is as expected from the base composition of ribosomal RNA, others show a large discrepancy which cannot be due to the presence of low G-C spacer-DNA. The crosshybridization observed with 1.3 and 0.7 X 100 molecular weight ribosomal RNAs and DNA, which varies greatly with different plant species, is not due to contamination of the ribosomal RNAs, and is specific for the ribosomal DNA of each species, probably largely restricted to those sequences coding for the two stable ribosomal RNAs. The double reciprocal plot may be used for the extrapolation of saturation values only with caution, because in these cases such plots are not linear over the whole of the hybridization reaction. stages of the reaction are therefore advantageous. The observation that the plot of reciprocal of hybridization against reciprocal of time fitted a straight line (9), which could be extrapolated to infinite time, suggested how saturation values could be obtained from measurements of the initial reaction. This should minimize problems of cross-hybridization. The use of this double reciprocal plot also eliminated the problem of RNA concentration-dependence of the saturation value, since analysis of the hybridization of complementary RNA and DNA from E. coli showed that the hybridization value obtained by extrapolation was essentially independent of the RNA concentration employed (9). This relationship has also been shown to hold for the hybridization of B. subtilis rRNA and DNA (6). The theoretical basis of this double reciprocal plot is not clear (9), but it represents a convenient, concentration-independent way of estimating a saturation value from the early part of the hybridization reaction.This paper describes some of the properties of the rRNA genes in higher plants and considers the nature of the crosshybridization observed between the 1.3 X 106 and 0.7 X 106 mol wt rRNAs. Some of the problems involved in the determination of a saturation value are discussed. Molecular hybridization of RNA to DNA has become an increasingly useful technique for studying many aspects of the genome. The stability and homogeneity of rRNA, which allows the preparation of highly purified molecules in relatively large amounts, has facilitated investigations of the rRNA genes by this method. The rRNA genes from Xenopus were the first genes to be isolated (8), and there is now a considerable knowledge of their fine structure (5, 7). Much less is known about rRNA genes in plants, and determination of the absolute redundancy of these genes is complicated by the apparent cross-hybridization between the two high molecular weight rRNAs (1.3 and 0.7 X 106) and their complementary sequences in the DNA, which confuses the determination of saturation levels (15). Similar cross-hybridization has been noted with the rRNAs from Escherichia coli (1, 4, 21), yeast (25), ra...

Environmentally induced changes in ribosomal RNA cistron number in flax

“…It has been shown that the extent of hybridisation depends on whether the 13 x 10° daltons rRNA or the 07 x 106 daltons rRNA are used (Scott and Ingle, 1973;Cullis, 1975). For the comparison of the various lines here a 2: 1 mixture of the two rRNAs was used as this gave essentially the same relative values as those obtained for the 13 x 106 daltons rRNA alone (Cullis, 1975).…”

Relation between ribosomal RNA genes and the DNA satellites of Phaseolus coccineus

“…The main band had a calculated value of 1.691 g/ml and they noticed two satellites at 1.696 and at 1.705 g/ml. INGLE et al (1973) found the values of 1.693, 1.692 and 1.694 g/ml for the main bands of coccineus, vulgaris and aureus respectively. The heavy satellite was 1.702, 1.703 and 1.705 in these same species respectively.…”

“…Other large satellites are present in Cucumis safivus (28 %), in C. melo (25 %) and in citrus species (24 to 19 %, INGLE et al 1973).…”

“…In plants a similar study has been delayed mainly due to difficulties in extracting DNA caused primarily by the presence of cellulose cell walls in these organisms. But plants are yielding other types of molecular information so far not avilable from animal tissues ( STERN and HOTTA 1969;AVANZI et al 1972, INGLE et al 1973SCOTT and INGLE 1973).…”

A novel ultrastructural feature of a gas-vacuolated Methanosarcina

“…DNA was extracted from the methanogen after disruption in a French Pressure Cell by the method of Scott and Ingle [11]. The guanine+ cytosine (G + C) content was estimated from buoyant density measurements by CsCI equilibrium density centrifugation using an MSE centriscan 75 centrifuge.…”