Pronounced structural similarities between the small subunit ribosomal RNA genes of wheat mitochondria and Escherichia coli.

Abstract

We present here the nucleotide sequence of the small subunit (18S) rRNA gene from wheat mitochondria. Aside from five discrete variable domains, this gene and the analogous (16S) rRNA gene in Escherichia coli show essentially a one-to-one correspondence in their potential secondary structures, with regions accounting for 86% of the bacterial 16S rRNA having a strict secondary structure counterpart in the mitochondrial 18S rRNA. Primary sequence identity between the two rRNAs ranges from 73% to 85% (76% overall) within regions of conserved secondary structure. Within a smaller secondary structure core common to all small subunit rRNAs, the wheat mitochondrial sequence shares substantially more primary sequence identity with the E. coli (eubacterial) sequence (88%) than with the small subunit rRNA sequences of Halobacterium vokcanii (an archaebacterium) (71%) or Xenopus laevis cytoplasm (61%). Moreover, the wheat mitochondrial sequence contains a very high proportion of certain lineage-specific residues that distinguish eubacterial/plastid 16S rRNAs from archaebacterial 16S and eukaryotic cytoplasmic 18S rRNAs. These data establish that the ancestry of the wheat mitochondrial 18S rRNA gene can be traced directly and specifically to the eubacterial primary kingdom, and the data provide compelling support for a relatively recent xenogenous (endosymbiotic) origin of plant mitochondria from eubacterialike organisms.The question of the origin of mitochondria continues to spark spirited debate (1, 2). Strong biochemical and structural similarities between mitochondria and bacteria lend support to an endosymbiotic (xenogenous) origin (3, 4) from organisms resembling, in respiratory metabolism (5) and cytochrome c structure (6), contemporary nonsulfur purple bacteria. On the other hand, mitochondrial genomes display patterns of gene arrangement and expression that differ substantially from characteristic bacterial patterns and that incorporate deviations from the universal genetic code (1, 2, 7). Such "nonbacterial" features, coupled with the fact that most of the "bacterial" traits of mitochondria are a manifestation of nuclear and not mitochondrial genes, have prompted a number of proposals (e.g., see refs. 8-10) that mitochondria had an autogenous origin, developing within a protoeukaryotic cell that gave rise to the genomes of both nucleus and organelles.In contrast to respiratory chain genes, rRNA genes are present in nuclear, bacterial (prokaryotic), and mitochondrial genomes, and considerable emphasis has been placed on rRNA sequence data as a means of distinguishing between autogenous and xenogenous hypotheses of mitochondrial origin (1, 2, 7). Moreover, since prokaryotes appear to be divided into two primary kingdoms, eubacteria and archaebacteria (11), any rRNA sequence data supporting a xenogenous origin should also indicate which of these two bacterial lineages has given rise to mitochondria. Sequence data supporting an endosymbiotic, eubacterial origin of wheat mitochondrial small subunit (18S) rRNA (...

The mitochondrial genome of an exsymbiotic Chlorella-like green alga

“…An analysis of ribosomal RNA (rRNA) sequences has raised the possibility that mt gehomes arose at different times during evolution, i.e., are polyphyletic [23]. Using 5S rRNA sequence comparisons between the nuclear genomes of algae and higher plants, Dayhoff and Schwartz [7] and Kuntzel et al [ 13] have postulated that mitochondria of green algae and vascular plants had an origin separate from mitochondria of fungi and animals.…”

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“…Once characterized as "too fantastic for present mention in polite biological society," this theory is now generally accepted, largely because of extensive compilations of supporting evidence (e.g., Margulis 1970Margulis , 1981Schwartz 785 and Dayhoff 1978;Baltscheffsky and Baltscheffsky 1981;Gray and Doolittle 1982;McCarroll et al 1983;Spenser et al 1984;Cavalier-Smith 1987a; Ballantyne and Chamberlin 1988;Gray 1992;Knoll 1992). Corresponding to such broad acceptance, a shift of emphasis is also occurring; presently, the focus is on questions of not merely whether mitochondria arose from endosymbionts, but how, and by what evolutionary mechanisms.…”

Nucleotide sequence and genome characterization of rice yellow mottle virus RNA

“…The serine (S) change in the catalytic site of RYMV and SBMV proteases in comparison to cellular and cysteine proteases of the other virus is shown in bold. (Chao et al,, 1983), the wheat mitochondrial 18S rRNA (Spencer et al, 1984) The 3" untranslated region of RYMV RNA contains 245 nt. Using the same computer program and algorithm, the last 147 nt of the RNA show a structure with a AG of 532.1,105 kJ with a pseudoknot domain (nt -92 to -147, relative to the 3" nucleotides) and a stem-loop domain (nt -1 to -91, relative to the 3' nucleotides) (Fig.…”