The sequences of the small ribosomal RNA gene and the phenylalanine tRNA gene are joined end to end in human mitochondrial DNA

Abstract

Summary The 5′ end proximal regions of the two HeLa cell mitochondrial rRNAs (16S and 12S) have been sequenced by partial enzymatic digestions of the 5′ end 32 P-labeled RNAs followed by electrophoretic fractionation of the products on polyacrylamide/urea gels. Likewise, a 600 nucleotide mitochondrial DNA (mit-DNA) fragment, previously designated Δ8a Hae , that contains the 5′ end of the 12S rRNA, has been sequenced by the method of Maxam and Gilbert. The first 71 nucleotides of the 12S rRNA and the DNA coding sequence have been aligned and found to be colinear. This observation extends to the 5′ end proximal segment of the 12S rRNA gene the conclusion of earlier experiments, indicating the absence of intervening sequences in the body of the small rRNA gene. A comparison of the 12S rRNA coding sequence determined here (286 nucleotides) with that of an E. coli 16S rRNA gene has revealed significant homologies. Previous electron microscopic analysis of hybrids between the heavy (H) strand of mit-DNA and ferritin-labeled mitochondrial 4S RNAs had shown the presence of a 4S RNA gene near the 5′ end of the 12S rRNA coding sequence. In the present work, a search of the DNA sequence for a cloverleaf structure has indeed revealed the occurrence of a tRNA Phe gene. The unexpected finding, however, has been that the 3′ end of this gene is contiguous to the 5′ end of the 12S rRNA coding sequence without any intervening nucleotides.