Bacterial L-cysteine biosynthesis via O-acetyl-L-serine

The taxon context denotes a reusable bacterial route, not universal possession or dependence. Sulfate uptake and reduction supply sulfide upstream. Thiosulfate incorporation into OAS produces S-sulfocysteine and requires downstream reduction, so it cannot silently replace the stated sulfide chemistry. CysK and CysM names describe diverse proteins: mycobacterial CysM prefers O-phosphoserine and CysO thiocarboxylate, whereas Campylobacter CysM uses OAS and strongly favors sulfide. Minor in-vitro side reactions do not automatically define a physiological module implementation. The cysteine synthase complex is regulatory context: CysE-tail occupation inhibits the CysK active site, and the enterobacterial interaction cannot be assumed in Pseudomonas or Bacillus. PTHR42811 grounds serine acetyltransferase; heterogeneous PTHR10314 contains both relevant OAS enzymes and proteins with other substrates/functions. PAINT node membership must therefore be combined with a gene review that establishes the route-specific core activity. The CysK node PTN000034535 includes bacterial CysK exemplars, but CysM node PTN008497439 also contains M. tuberculosis P9WP53; it cannot alone qualify a native OAS-route implementation. Absence of a representative from the cached tree is not evidence for absence of family membership. Campylobacter Cj0912c is retained as an explicitly unplaced OAS-active alternative: its CysM symbol and CysK-like signature do not establish either existing PAINT node. Fifteen positive member exemplars span E. coli, Pseudomonas, Bacillus, Campylobacter and Mycobacterium; a sixteenth gene review assesses Mtb CysM as a mechanistically distinct comparator. Mtb regulatory-complex formation has direct support in PMID:38311199, but exact full-length construct and assembly details remain unresolved; its CysE activity is reduced, so regulatory direction is not universal. Bacillus instead has directly demonstrated CymR-CysK regulation, while tested CysE-CysK association is negative. Quoted biochemical results, genetic evidence, family inference and construct/strain limits are distinguished. An intact enzyme panel describes supported capacity, not measured native flux or whole-genome completeness.

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