Asparagine degradation in Rhizobium etli

Abstract

The degradation of asparagine by Rhizobium etli involves asparaginase and aspartate ammonia-lyase (L-aspartase). The two enzymes were shown to be positively regulated by asparagine and negatively regulated by the carbon source. Asparaginase activity was not regulated by oxygen concentration or by nitrogen catabolite repression. Induction of both enzymes by asparagine enables R. etli to utilize asparagine as carbon source. Asparaginase may also be involved in maintaining the optimal balance between asparagine and aspartate. Aspartase was not involved in the utilization of aspartate or glutamate as carbon source. The presence of high levels of the two enzymes in R. etli bacteroids suggests that they may have a role in symbiosis between R. etli and Phaseolus vulgaris.

Isolation and characterization of Rhizobium etli mutants altered in degradation of asparagine

“…The second enzyme, L-aspartase (EC 4.3.1.1), catalyzes the reversible deamination of L-aspartate to yield fumarate and ammonium. R. etli asparaginase is found to be positively regulated by its substrate asparagine and negatively regulated by the carbon source; it is not regulated by the amount of oxygen dissolved in the growth medium or by nitrogen catabolite repression, and some asparaginase activity is detected when R. etli is grown on ammonium as a nitrogen source and succinate as a carbon source (20). Asparaginase has been studied in other gramnegative bacteria such as Escherichia coli (10,12,22,24,25,37,47), Salmonella enterica (23), Erwinia chrysanthemi (19), and Vibrio proteus (41) and in gram-positive bacteria such as Bacillus subtilis (1,21,43) and Staphylococcus aureus (35,42).…”

“…In R. etli, L-asparagine can be utilized as the sole carbon and nitrogen source through the action of two enzymes (20). The first enzyme, asparaginase (EC 3.5.1.1), catalyzes the hydrolysis of L-asparagine to L-aspartate and ammonium.…”

“…etli aspartase was found to be positively regulated by asparagine, negatively regulated by the carbon source and by ammonium, and not regulated by the amount of oxygen dissolved in the growth medium (20). L-Aspartase has been studied in other bacteria such as E. coli (25,27,33,36), B. subtilis (43), Pseudomonas fluorescens (30,45), and Serratia marcescens (44).…”

“…Mutants of R. etli wild-type strain CFN42 (39) were obtained by general Tn5 mutagenesis with plasmid pSUP5011 carrying transposon Tn5-Mob (40). Kanamycin transconjugants were isolated on PY medium (20) containing this antibiotic and nalidixic acid. About 20,000 kanamycinresistant transconjugants were obtained and then collected on PY medium.…”

“…Batch cultures of R. etli were grown as described elsewhere (20). The nitrogen and carbon sources were used at 10 mM.…”