Nucleic Acids Research, 1992, Vol. 20, No. 9 2349-2353
© 1992
MOLECULAR BIOLOGY |
Mutational evidence for competition between the P1 and the P10 helices of a mitochondrial group I intron
Department of Immunology and Medical Microbiology, University of Florida College of Medicine Box 100266 Gainesville, FL 32610, USA
*To whom correspondence should be addressed
Received December 31, 1991. Revised March 27, 1992. Accepted March 27, 1992.
A guanosine to cytosine transversion at position 2 of the fifth intron of the mitochondrial gene COB blocks the ligation step of splicing. This mutation prevents the formation of a base pair within the P1 helix of this group I intron—the RNA duplex formed between the 3' end of the upstream exon and the internal guide sequence. The mutation also reduces the rate of the first step of splicing (guanosine addition at the 5' splice junction) while stimulating hydrolysis at the 3' intron-exon boundary. Consequently, the ligation of exons is blocked because the 3' exon is removed prior to cleavage at the 5' splice junction. The lesion can be suppressed by second-site mutations that preserve the potential for base-pairing at this position. Because the P1 duplex and the P10 duplex (between the guide sequence and the 3' exon) overlap at the affected position, these results imply that the P1 and P10 pairings represent alternative structures that do not, indeed cannot, form simultaneously.
+ Present address: Department of Plant Pathology, Institute for Food and Agricultural Sciences, University of Florida, Gainesville, FL 32611, USA