Maize Mitochondria: Purification and Characterization of Ribosomes and Ribosomal Ribonucleic Acid

Abstract

Mitochondria were prepared from etiolated maize shoots (Zea nuays L. var. McNair 508) by homogenization followed by differential centrifugation and equilibrium banding in discontinuous sucrose or Renografin-sucrose gradients. Mitochondria prepared by sucrose banding showed better physiological integrity than those prepared by Renografin-sucrose banding, although both procedures yielded mitochondria that showed respiratory control and coupling of oxidation to phosphorylation of ADP. Mitochondria prepared by Renografin-sucrose banding were free of dectectable cytoplasmic ribosomal RNA, while sucrose banding resulted in a low level of contamination. Ribosomes isolated from mitochondria sedimented at about 78S, with subunits sedimenting at 60 and 44S. Using Escherichia coli ribosomal RNA as internal standards, the molecular weights of mitochondrial ribosomal RNAs were found to be 0.74 to 0.75 and 1.26 X 106 daltons by polyacrylamide gel electrophoresis, before or after denaturation in formaldehyde. Cytoplasmic ribosomal RNA molecular weights were 0.70 and 1.26 X 168 before denaturation, and 0.68 and 1.5 X 106 after denaturation, suggesting an unusual reaction of the heavy ribosomal RNA to formaldehyde.The literature concerning the biogenesis, properties, and interrelationships of mitochondria and other cellular organelles has been extensively reviewed in recent years (1,4,5,23). Similarities of mitochondria to bacteria and other prokaryotes have long been used to suggest an evolutionary relationship between the latter and the endosymbiotic mitochondria (1). Examination of some of the constituents of mitochondria, however, has shown marked differences between bacteria and mitochondria, and among mitochondrial sources. Of particular interest is the observation that mitochondria from a number of sources possess ribosomes and ribosomal RNA that are distinct from bacterial ribosomes in certain characteristics. The sedimentation coefficients of mitochondrial ribosomes range from 55S in mammalian tissue (22) plant mitochondrial ribosomes. Maize mitochondrial ribosomes have been described as sedimenting at 66S (27), and a low guanine-cytosine content of soybean mitochondrial ribosomal RNA has been reported (2). The latter characteristic is common among mitochondrial RNAs (5).As a part of investigations on cytoplasmically inherited phenomena in maize, this study was initiated to purify maize mitochondria and to characterize constituents of the organelle. The present report describes a purification procedure that frees mitochondria of apparent cytoplasmic ribosomal RNA and presents evidence of a high mol wt of maize mt-rRNA.' Ribosomes from the preparations showed sedimentation characteristics similar to those of cytoplasmic ribosomes. While this study was in progress, Leaver and Harmey (16,17) found similar data for mitochondria from a range of higher plants. They reported a 78S ribosome, and ribosomal RNA mol wt of 0.69 to 0.78 and 1.12 to 1.18 X 106 for five dicotyledonous species. The data reported herein substan...

The Arabidopsis Cytosolic Ribosomal Proteome: From form to Function

“…However, given that the 70S and 80S ribosomes of C. reinhardtii sedimented closely on sucrose gradients (Yamaguchi et al, 2003) and that mitochondrial ribosomes from higher plants have been observed to sediment anywhere between 70S (Vasconcelos and Bogorad, 1971; Pinel et al, 1986) and 78S (Leaver and Harmey, 1973, 1976; Pring, 1974), the above observations highlight the importance of early fractionation steps, orthogonal to sucrose gradient purification, in obtaining pure cytosolic ribosomes required for confident discrimination of cytosolic and organellar ribosomal proteomes. In this author’s view, this technical point is worth highlighting given the potential functional and evolutionary significance of parallel-targeting of r-proteins to multiple ribosomes in eukaryotic cells and the fact that just a few simple protocol modifications could greatly enhance the utility of future studies in addressing this important possibility.…”

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Variation in mitochondrial translation products associated with male-sterile cytoplasms in maize

“…This article must therefore be hereby marked "advertisement" in accordance with 18 U. S. C. §1734 solely to indicate this fact. 3841 distinct protein synthesizing system (14,15) which makes a limited range of polypeptides (16,17), similar in both number and molecular-weight range to those synthesized by mitochondria from Ascomycetes and animal cells (18). In plants these polypeptides remain to be identified, but it is likely, by analogy with other cell types, that many of them are components of enzyme complexes located in the inner mitochondrial membrane (18).…”