Abstract
Histophilus somni (
Haemophilus somnus) is an important pathogen of cattle that is responsible for respiratory disease, septicemia, and systemic diseases such as thrombotic meningoencephalitis, myocarditis, and abortion. A variety of virulence factors have been identified in
H. somni, including compositional and antigenic variation of the lipooligosaccharide (LOS). Phosphorylcholine (ChoP) has been identified as one of the components of
H. somni LOS that undergoes antigenic variation. In this study, five genes (
lic1ABCD
Hs
and
glpQ) with homology to genes responsible for ChoP expression in
Haemophilus influenzae LOS were identified in the
H. somni genome. An
H. somni open reading frame (ORF) with homology to
H. influenzae lic1A (
lic1A
Hi
) contained a variable number of tandem repeats (VNTR). However, whereas the tetranucleotide repeat 5′-CAAT-3′ is present in
lic1A
Hi
, the VNTR in
H. somni lic1A (
lic1A
Hs
) consisted of 5′-AACC-3′. Due to the propensity of VNTR to vary during replication and cause the ORF to shift in and out of frame with the upstream start codon, the VNTR were deleted from
lic1A
Hs
to maintain the gene constitutively on. This construct was cloned into
Escherichia coli, and functional enzyme assays confirmed that
lic1A
Hs
encoded a choline kinase, and that the VNTR were not required for expression of a functional gene product. Variation in the number of VNTR in
lic1A
Hs
correlated with antigenic variation of ChoP expression in
H. somni strain 124P. However, antigenic variation of ChoP expression in strain 738 predominately occurred through variable extension/truncation of the LOS outer core. These results indicated that the
lic1
Hs
genes controlled expression of ChoP on the LOS, but that in
H. somni there are two potential mechanisms that account for antigenic variation of ChoP.