A REAPPRAISAL OF THE DIVERSITY AND CLASS DISTRIBUTION OF ASPARTATE TRANSCARBAMOYLASES IN GRAM-NEGATIVE BACTERIA

Recently, the subunit composition of class A aspartate transcarbamoyla ses (ATCases) in fluorescent pseudomonads has been clarified. We prese nt evidence that distribution of this type of ATCase may be more wides pread 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 b y Bacillus subtilis ATCase (similar to 100 kDa), Using gradient gel el ectrophoresis with activity-staining to scan bacterial sonicates, we r eport the existence of six more class A ATCases. We have purified one of these, Acinetobacter calcoaceticus ATCase, and found its subunit co mposition to be similar to that of the pseudomonad ATCases. Two of the se ATCases come from bacteria outside the gamma-subgroup of the Proteo bacteria, one from the a-subgroup and one from Deinococcus radiophilus , a species phylogenetically remote from the Proteobacteria. Unexpecte dly, three bacterial species, closely related to the fluorescent pseud omonads and acinetobacters, have ATCases of 100 kDa (class C). One of these, Stenotrophomonas (formerly Xanthomonas) maltophilia has been pu rified and found to be a homotrimer of 35 kDa polypeptide chains. We b elieve this is the first time that class C ATCases have been reported in Cram-negative bacteria. A distinctive cluster in the gamma-3 subgro up of the Proteobacteria is formed by the enteric bacteria and their r elatives. So far only class B ATCases have been reported in this group . The evolutionary implications of these findings are discussed.