Abstract
The bacteriophage T4 segA gene lies in a genetically unmapped region between the gene beta gt (beta-glucosyltransferase) and uvsX (recombination protein) and encodes a protein of 221 amino acids. We have found that the first 100 amino acids of the SegA protein are highly similar to the N termini of four other predicted T4 proteins, also of unknown function. Together these five proteins, SegA-E (similar to endonucleases of group I introns), contain regions of similarity to the endonuclease I-Tev I, which is encoded by the mobile group I intron of the T4 td gene, and to putative endonucleases of group I introns present in the mitochondria of Neurospora crassa, Podospora anserina, and Saccharomyces douglasii. Intron-encoded endonucleases are required for the movement (homing) of the intron DNA into an intronless gene, cutting at or near the site of intron insertion. Our in vitro assays indicate that SegA, like I-Tev I, is a Mg(2+)-dependent DNA endonuclease that has preferred sites for cutting. Unlike the I-Tev I gene, however, there is no evidence that segA (or the other seg genes) resides within introns. Thus, it is possible that segA encodes an endonuclease that is involved in the movement of the endonuclease-encoding DNA rather than in the homing of an intron.