The thyA gene from Bacillus subtilis exhibits similarity with the phage phi 3T thymidylate synthase gene.

The gene encoding thymidylate synthase A (thyA) was cloned from a genomic library of Bacillus subtilis 168. The sequence of thyA was found to be highly similar to that of the phage phi 3T thymidylate synthase gene. This similarity is, however, limited to about 862 nucleotides, spanning the coding region and the adjacent 3' region. The flanking sequences are not related. An integrative plasmid containing a DNA fragment with a deletion within the coding region of thyA was constructed and used to replace the chromosomal thyA gene. Transformants unable to grow without thymidine at 47 degrees C were obtained. Genetic and physical mapping techniques were used to show that the cloned DNA fragment harbouring delta thyA was integrated at 168 degrees on the B. subtilis chromosome.