To determine whether in Azotobacter vinelandii the P-II protein influe
nces the regulation of nif gene expression in response to fluxes in th
e ammonium supply, the gene encoding P-II was isolated sand characteri
zed. Its deduced translation product was highly similar to P-II protei
ns from other organisms, with the greatest degree of relatedness being
exhibited to the Escherichia coli glnK gene product. A gene designate
d amtB was found downstream of and was cotranscribed with glnK as in E
. coli. The AmtB protein is similar to functionally characterized ammo
nium transport proteins from a few other eukaryotes and one other prok
aryote, glnK and amtB comprise an operon. Attempts to isolate a stable
glnK mutant strain were unsuccessful, suggesting that glnK, like glnA
, is an essential gene in A. vinelandii. amtB mutants were isolated, a
nd although growth on limiting amounts of ammonium was similar in the
mutant and wild-type strains, the mutants were unable to transport [C-
14] methylammonium.