The methyl-branched fatty acyl components of
sulfolipid-I (SL-I), a major
glycolipid of the human pathogen Mycobacterium tuberculosis, are synthesized by the
polyketide synthase Pks2. Rv3824c (papA1), located downstream of pks2, encodes a
protein that belongs to a subfamily of
acyltransferases associated with mycobacterial
polyketide synthases [
polyketide synthase-associated
proteins (PAPs)]. The presence of a conserved
acyltransferase motif (HX(3)DX(14)Y) suggested a role for PapA1 in acylation of sulfated
trehalose to form SL-I. Targeted deletion of the H37Rv papA1 resulted in loss of SL-I, demonstrating its role in mycobacterial sulfolipid biosynthesis. Furthermore, SL-I synthesis was restored in the mutant strain following complementation with papA1, but not with mutant alleles of papA1 containing alterations of key residues in the
acyltransferase motif, confirming that PapA1 was an
acyltransferase. While other M.
tuberculosis pks clusters are associated with a single PAP-encoding gene, it was demonstrated that another open reading frame, Rv3820c (
papA2), located 5.8 kb downstream of papA1 is also an
acyltransferase gene involved in SL-I biosynthesis: deletion of
papA2 abolished SL-I production. The absence of any partially acylated intermediates in either null mutant indicated that both PapA1 and
PapA2 were required for all acylation steps of SL-I assembly.