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Nucleic Acids Research, 1991, Vol. 19, No. 23 6491-6497
© 1991

MOLECULAR BIOLOGY

A mixed group II/group III twintron in the Euglena gracilis chloroplast ribosomal protein S3 gene: evidence for intron insertion during gene evolution

Donald W. Copertino1, David A. Christopher1,+ and Richard B. Hallick1,2,*

1Departments of Molecular and Cellular Biology, University of Arizona AZ 85721, USA 2Departments of Biochemistry, University of Arizona Tucson, AZ 85721, USA

*To whom correspondence should be addressed

Received August 20, 1991. Accepted November 4, 1991.

The splicing of a 409 nucleotide intron from the Euglena gracilis chloroplast ribosomal protein S3 gene (rps3) was examined by cDNA cloning and sequencing, and northern hybridization. Based on the characterization of a partially spliced pre-mRNA, the intron was characterized as a ‘mixed’ twintron, composed of a 311 nucleotide group II intron internal to a 98 nucleotide group III intron. Twintron excision is via a 2-step sequential splicing pathway, with removal of the internal group II intron preceding excision of the external group III intron. Based on secondary structural analysis of the twintron, we propose that group III introns may represent highly degenerate versions of group II introns. The existence of twintrons is interpreted as evidence that group II introns were inserted during the evolution of Euglena chloroplast genes from a common ancestor with eubacteria, archaebacteria, cyanobacteria, and other chloroplasts.

+Present address: Department of Biochemistry and Biophysics. Texas A&M University, College Station, TX 77843, USA


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