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Genetics and Molecular Biology |
Department of Biological Chemistry, University of Copenhagen, DK-1307 Copenhagen K, Denmark1
Department of Microbiology, Technical University of Denmark, DK-2800 Lyngby, Denmark2
Department of Genetics, Trinity College, Dublin 2, Ireland3
Author for correspondence: Per Nygaard. Tel: +45 35322005. Fax: +45 35322040. e-mail: nygaard{at}mermaid.molbio.ku.dk
Bacillus subtilis can utilize the purine bases adenine, hypoxanthine and xanthine as nitrogen sources. The utilization of guanine as a nitrogen source is reported here. The first step is the deamination of guanine to xanthine catalysed by guanine deaminase (GDEase). To isolate mutants defective in GDEase activity, a collection of mutant strains was screened for strains unable to use guanine as a nitrogen source. The strain BFA1819 (yknA) showed the expected phenotype and no GDEase activity could be detected in this strain. A new name for yknA, namely gde, is proposed. The gde gene encodes a 156 amino acid polypeptide and was preceded by a promoter sequence that is recognized by the 
A form of RNA polymerase. High levels of GDEase were found in cells grown with purines and intermediary compounds of the purine catabolic pathway as nitrogen sources. Allantoic acid, most likely, is a low molecular mass inducer molecule. The level of GDEase was found to be subjected to global nitrogen control exerted by the GlnA/TnrA-dependent signalling pathway. The two regulatory proteins of this pathway, TnrA and GlnR, indirectly and positively affected gde expression. This is the first instance of a gene whose expression is positively regulated by GlnR. The GDEase amino acid sequence shows no homology with the mammalian enzyme. In agreement with this are the different physiological roles for the two enzymes.
Keywords: guanine deaminase, purine catabolism, nitrogen metabolism
Abbreviations: GDEase, guanine deaminase; XDHase, xanthine dehydrogenase
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