Abstract
An invertible DNA element of 6.8 kb, designated the hsd1 locus, was identified in the chromosome of Mycoplasma pulmonis. Infection of host cells with mycoplasma virus P1 revealed that the organism's restriction and modification (R‐M) properties are controlled by inversion of hsd1. The nucleotide sequence of hsd1 revealed several genes, the predicted amino acids of which bear striking similarity to the subunits of the type I R‐M enzymes previously found only in enteric bacteria.