| Summary: | <p>Abstract</p> <p>Background</p> <p>Methylthioadenosine, the main by-product of spermidine synthesis, is degraded in <it>Bacillus subtilis</it> as adenine and methylthioribose. The latter is an excellent sulfur source and the precursor of quorum-sensing signalling molecules. Nothing was known about methylthioribose recycling in this organism.</p> <p>Results</p> <p>Using trifluoromethylthioribose as a toxic analog to select for resistant mutants, we demonstrate that methylthioribose is first phosphorylated by MtnK, methylthioribose kinase, the product of gene <it>mtnK</it> (formerly <it>ykrT</it>), expressed as an operon with <it>mtnS</it> (formerly <it>ykrS</it>) in an abundant transcript with a S-box leader sequence. Although participating in methylthioribose recycling, the function of <it>mtnS</it> remained elusive. We also show that MtnK synthesis is boosted under starvation condition, in the following decreasing order: carbon-, sulfur- and nitrogen-starvation. We finally show that this enzyme is part of the family Pfam 01633 (choline kinases) which belongs to a large cluster of orthologs comprizing antibiotic aminoglycoside kinases and protein serine/threonine kinases.</p> <p>Conclusions</p> <p>The first step of methylthioribose recycling is phosphoryltaion by MTR kinase, coded by the <it>mtnK</it> (formerly <it>ykrT</it>) gene. Analysis of the neighbourhood of <it>mtnK</it> demonstrates that genes located in its immediate vicinity (now named <it>mtnUVWXYZ</it>, formerly <it>ykrUVWXYZ</it>) are also required for methylthioribose recycling.</p>
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