Related homing endonucleases I-BmoI and I-TevI use different strategies to cleave homologous recognition sites.
A typical homing endonuclease initiates mobility of its group I intron by recognizing DNA both upstream and downstream of the intron insertion site of intronless alleles, preventing the endonuclease from binding and cleaving its own intron-containing allele. Here, we describe a GIY-YIG family homing endonuclease, I-BmoI, that possesses an unusual recognition sequence, encompassing 1 base pair upstream but 38 base pairs downstream of the intron insertion site. I-BmoI binds intron-containing and intronless substrates with equal affinity but can nevertheless discriminate between the two for cleavage. I-BmoI is encoded by a group I intron that interrupts the thymidylate synthase (TS) gene (thyA) of Bacillus mojavensis s87-18. This intron resembles one inserted 21 nucleotides further downstream in a homologous TS gene (td) of Escherichia coli phage T4. I-TevI, the T4 td intron-encoded GIY-YIG endonuclease, is very similar to I-BmoI, but each endonuclease gene is inserted within a different position of its respective intron. Remarkably, I-TevI and I-BmoI bind a homologous stretch of TS-encoding DNA and cleave their intronless substrates in very similar positions. Our results suggest that each endonuclease has independently evolved the ability to distinguish intron-containing from intronless alleles while maintaining the same conserved recognition sequence centered on DNA-encoding active site residues of TS.
Edgell
D R
DR
Department of Biological Sciences and Center for Molecular Genetics, State University of New York, Albany, NY 12222, USA.
Shub
D A
DA
D000595:Amino Acid Sequence; D001407:Bacillus / Q000201:enzymology / Q000235:genetics; D004706:Endodeoxyribonucleases / Q000235:genetics*; D004926:Escherichia coli / Q000201:enzymology / Q000235:genetics; D019143:Evolution, Molecular; D007438:Introns / Q000235:genetics*; D008969:Molecular Sequence Data; D016415:Sequence Alignment; D013379:Substrate Specificity
D004706:Endodeoxyribonucleases; C432135:I-BmoI endonuclease; C065074:I-TEVI endonuclease