A reappraisal of the diversity and class distribution of aspartate transcarbamoylases in gram-negative bacteria

Recently, the subunit composition of class A aspartate transcarbamoylases (ATCases) in fluorescent pseudomonads has been clarified. We present evidence that distribution of this type of ATCase may be more widespread than at first suspected. Bacterial ATCases exist in three forms: class A (molecular mass similar to 450-500 kDa); class B, typified by Escherichia coli ATCase (similar to 300 kDa); and class C, typified by Bacillus subtilis ATCase (similar to 100 kDa), Using gradient gel electrophoresis with activity-staining to scan bacterial sonicates, we report the existence of six more class A ATCases. We have purified one of these, Acinetobacter calcoaceticus ATCase, and found its subunit composition to be similar to that of the pseudomonad ATCases. Two of these ATCases come from bacteria outside the gamma-subgroup of the Proteobacteria, one from the a-subgroup and one from Deinococcus radiophilus, a species phylogenetically remote from the Proteobacteria. Unexpectedly, three bacterial species, closely related to the fluorescent pseudomonads and acinetobacters, have ATCases of 100 kDa (class C). One of these, Stenotrophomonas (formerly Xanthomonas) maltophilia has been purified and found to be a homotrimer of 35 kDa polypeptide chains. We believe this is the first time that class C ATCases have been reported in Cram-negative bacteria. A distinctive cluster in the gamma-3 subgroup of the Proteobacteria is formed by the enteric bacteria and their relatives. So far only class B ATCases have been reported in this group. The evolutionary implications of these findings are discussed.