COMPARISON STUDIES OF DINITROGENASE REDUCTASE ADP-RIBOSYL TRANSFERASEDINITROGENASE REDUCTASE ACTIVATING GLYCOHYDROLASE REGULATORY SYSTEMS IN RHODOSPIRILLUM-RUBRUM AND AZOSPIRILLUM-BRASILENSE

Reversible ADP ribosylation of dinitrogenase reductase, catalyzed by t he dinitrogenase reductase ADP-ribosyl transferase (DRAT)/dinitrogenas e reductase activating glycohydrolase (DRAG) regulatory system, has be en characterized in both Rhodospirillum rubrum and Azospirillum brasil ense. Although the general functions of DRAT and DRAG are very similar in these two organisms, there are a number of interesting differences , e.g., in the timing and extent of the regulatory response to differe nt stimuli. In this work, the basis of these differences has been stud ied by the heterologous expression of either draTG or nifH from A. bra silense in R. rubrum mutants that lack these genes, as well as the exp ression of draTG from R. rubrum in an A. brasilense draTG mutant. In g eneral, these hybrid strains respond tb stimuli in a manner similar to that of the wild-type parent of the recipient strain rather than the wild-type source of the introduced genes. These results suggest that t he differences seen in the regulatory response in these organisms are not primarily a result of different properties of DRAT, DRAG, or dinit rogenase reductase. Instead, the differences are likely the result of different signal pathways that regulate DRAG and DRAT activities in th ese two organisms, Our results also suggest that draT and draG are cot ranscribed in A. brasilense.