Description:
<jats:title>Summary</jats:title><jats:p> <jats:italic>Streptococcus agalactiae</jats:italic>
(GBS) is a major cause of serious newborn bacterial infections. Crucial to GBS evasion of host immunity is the production of a capsular polysaccharide (CPS) decorated with sialic acid, which inactivates the alternative complement pathway. The CPS operons of serotypes Ia and III GBS have been described, but the CPS sialyltransferase gene was not identified. We identified
<jats:italic>cpsK</jats:italic>
, an open reading frame in the CPS operon of most serotypes, which was homologous to the lipooligosaccharide (LOS) sialyltransferase gene,
<jats:italic>lst</jats:italic>
, of
<jats:italic>Haemophilus ducreyi</jats:italic>
. To determine if
<jats:italic>cpsK</jats:italic>
might encode a sialyltransferase, we complemented a
<jats:italic>H. ducreyi lst</jats:italic>
mutant with
<jats:italic>cpsK</jats:italic>
. CpsK was expressed in
<jats:italic>H. ducreyi</jats:italic>
and LOS was isolated and analysed for sialic acid content by SDS–PAGE and high‐performance liquid chromatography (HPLC). Sialo‐LOS was seen in the wild‐type,
<jats:italic>cpsK‐</jats:italic>
or
<jats:italic>lst</jats:italic>
‐complemented mutant strains, but not in the mutant without
<jats:italic>cpsK</jats:italic>
. Addition of Neu5Ac to the LOS was confirmed by mass spectro‐scopy. Lectin binding studies detected terminal Neu5Ac(
<jats:bold>α</jats:bold>
2
<jats:bold>→</jats:bold>
3)Gal
<jats:bold>(β</jats:bold>
1‐ on LOS produced by the wild‐type,
<jats:italic>cpsK</jats:italic>
or
<jats:italic>lst</jats:italic>
‐complemented mutant strain LOS, compared with the mutant alone. Our data charac‐terize the first sialyltransferase gene from a Gram‐ positive bacterium and provide compelling evidence that its product catalyses the
<jats:bold>α</jats:bold>
2,3 addition of Neu5Ac to
<jats:italic>H. ducreyi</jats:italic>
LOS and therefore the terminal side‐chain of GBS CPS. Phylogenetic studies further indicated that
<jats:italic>lst</jats:italic>
and
<jats:italic>cpsK</jats:italic>
are related but distinct from sialyltransferases of most other bacteria and, along with their similar codon usage bias and G
<jats:bold>+</jats:bold>
C content, suggests acquisition by lateral transfer from an ancestral low G
<jats:bold>+</jats:bold> C organism.</jats:p>