Genes encoding thymidylate synthases A and B in the genus Bacillus are members of two distinct families.

Bacillus subtilis strain 168 is known to possess two genes that encode thymidylate synthases, thyA and thyB. We have identified genes similar to the thyA and thyB genes in several Bacillus strains by Southern hybridization and by DNA amplification with sequence-specific primers. Analysis of thyA genes cloned from B. subtilis W23 strain 2A6, B. subtilis ATCC6633, B. amyloliquefaciens S18 and B. atrophaeus S223 reveals that they are very similar to the thyA genes from B. subtilis 168 and its phage phi3T, but differ considerably from the majority of known prokaryotic and eukaryotic thymidylate synthases.