Abstract
We have recently described three group I introns inserted into a single gene,
orf142
, of the staphylococcal bacteriophage Twort and suggested the presence of at least two additional self-splicing introns in this phage genome. Here we report that two previously uncharacterized introns, 429 and 1087 nt in length, interrupt the Twort gene coding for the large subunit of ribonucleotide reductase (
nrdE
). Reverse transcription–polymerase chain reaction (RT–PCR) of RNA isolated from
Staphylococcus aureus
after phage infection indicates that the introns are removed from the primary transcript
in vivo
. Both
nrdE
introns show sequence similarity to the Twort
orf142
introns I2 and I3, suggesting either a common origin of these introns or shuffling of intron structural elements. Intron 2 encodes a DNA endonuclease, I-
Two
I, with similarity to homing endonucleases of the HNH family. Like I-
Hmu
I and I-
Hmu
II, intron-encoded HNH endonucleases in
Bacillus subtilis
phages SPO1 and SP82, I-
Two
I nicks only one strand of its DNA recognition sequence. However, whereas I-
Hmu
I and I-
Hmu
II cleave the template strand in exon 2, I-
Two
I cleaves the coding strand in exon 1. In each case, the 3′ OH created on the cut strand is positioned to prime DNA synthesis towards the intron, suggesting that this reaction contributes to the mechanism of intron homing. Both
nrdE
introns are inserted in highly conserved regions of the ribonucleotide reductase gene, next to codons for functionally important residues.