we use low external biotin concentrations (10-100 pM) to show that a R. etli bio M mutant had significantly retaining it (guillen navarro, submitted ) . Overexpression of bio Y in wild-type S. meliloti Rm1021 allows better than wild-type groeth in medias supplemented with dethiobiotin. It was suggested that Bio Y migth play a role in converting dethiobiotin to biotin by a mechanism distinct from that of a classical biotin synthase.