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

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Abstract
Nucleotide sequence of the small subunit (18S) rRNA gene from wheat mitochondria is presented. Aside from 5 discrete variable domains, this gene and the analogous (16S) rRNA gene in E. coli show essentially a 1-to-1 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 2 rRNA ranges from 73-85% (76% overall) within regions of conserved secondary structure. Within a smaller secondary structure core common to all small subunit rRNA, 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 volcanii (an archaebacterium) (71%) or Xenopus laevis cytoplasm (61%). The wheat mitochondrial sequence contains a very high proportion of certain lineage-specific residues that distinguish eubacterial/plastid 16S rRNA from archaebacterial 16S and eukaryotic cytoplasmic 18S rRNA. The ancestry of the wheat mitochondrial 18S rRNA gene can be traced directly and specifically to the eubacterial primary kingdom. Data suggest a relatively recent xenogenous (endosymbiotic) origin of plant mitochondria from eubacteria-like organisms.