A novel locus of Yersinia enterocolitica serotype O:3 involved in lipopolysaccharide outer core biosynthesis

Yersinia enterocolitica serotype O:3 strain 6471/76‐c (YeO3‐c) was sensitive to bacteriophage φR1‐37 when grown at 37°C but not when grown at 22°C because of steric hindrance by abundant lipopolysaccharide (LPS) O‐side chain (O‐antigen) expressed at 22°C. The transposon library of YeO3‐c was grown at 37°C and screened for phage φR1‐37‐resistant transposon insertion mutants. Three types of mutant were isolated: (i) phage receptor mutants expressing O‐antigen (LPS‐smooth), (ii) phage receptor mutants not expressing O‐antigen (LPS‐rough), and (iii) LPS‐smooth mutants with the phage receptor constitutively sterically blocked. Mutant type (i) was characterized in detail; the transposon insertion inactivates an operon, named the trs operon. The main findings based on this mutant are: (i) the trs operon is involved in the biosynthesis of the LPS outer core in YeO3‐c; the nucleotide sequence of the trs operon revealed eight novel genes showing similarity to known polysaccharide biosynthetic genes of various Gram‐negative bacteria as well as to capsule biosynthesis genes of Staphylococcus aureus; (ii) the biosynthesis of the core of YeO3‐c involves at least two genetic loci; (iii) the trs operon is required for the biosynthesis of the bacteriophage φR1‐37 receptor structures; (iv) the homopolymeric O‐antigen of YeO3‐c is ligated to the inner core in Y. enterocolitica O:3; (v) the trs operon is located between the adk—hemH and galE—gsk gene pairs in the Y. enterocolitica chromosome; and (vi) the phage φR1‐37 receptor is present in many but not in all Y. enterocolitica serotypes. The results also allow us to speculate that the trs operon is a relic of the ancestral rfb region of Y. enterocolitica O:3 carrying genes indispensable for the completion of the core polysaccharide biosynthesis.