SUMMARY: FOR mutants of are resistant to Felix O phage, whose receptor includes the -acetylglucosamine branch of the lipopolysaccharide (LPS) core, but smooth in cultural properties, antigenic character and phage sensitivity pattern (MacPhee , 1975). The (FOR) genes determining the FOR character of nine mutants were transduced into a smooth recipient: the nine FOR transductants (and a tenth FOR mutant) were then made (i.e. unable to make O chains) by transduction or Hfr crosses. The FOR strains were sensitive to FO phage but nearly all of them showed a somewhat reduced efficiency of plating and diminished rate of adsorption of the phage. This observation and the Ra (complete core) serological activity of their LPS (tested by haemagglutination inhibition) indicate the presence of some, but less than the normal number of, completed core chains in FOR LPS. On the basis of the sensitivities of the FOR transductants and their derivatives to various “rough-specific” phages, their increased sensitivities to some antibiotics and to deoxycholate and the serological activity of the FOR LPS in various incomplete core systems, the mutants were divided into three groups: (i) five mutants with probable defects in previously undetected gene(s) concerned with formation of both the galactose I and the galactose II units of the LPS core; (ii) two mutants with defects inferred to affect the structure of the inner part of the core and also interfere with addition of the -acetylglucosamine branch; (iii) three mutants in which no type of incomplete core could be detected, probably affected in formation of the inner part of the core chain. The mutation of one mutant of the last class, unlike those of the other nine mutants tested, lay outside the segment, in the 90 to 116 min region of the linkage map.


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