Genomic organization and expression of the expanded SCG/L/R gene family of Leishmania major: internal clusters and telomeric localization of SCGs mediating species-specific LPG modifications.

Stage-specific modifications to the abundant surface lipophosphoglycan (LPG) adhesin of Leishmania play critical roles in binding and release of the parasite during its infectious cycle in the sand fly, and control the ability of different fly species to transmit different parasite strains and species. In Leishmania major Friedlin V1, binding to a sand fly midgut lectin is mediated by side chain galactosyl (scGal) modifications of the LPG phosphoglycan (PG) repeats, while release occurs following arabinose-capping of scGals. Previously we identified a family of six SCG genes encoding PG scbeta-galactosyltransferases, and here we show that the extended SCG gene family (now termed SCG/L/R) encompasses 14 members in three subfamilies (SCG, SCGL and SCGR). Northern blot and RT-PCR analyses suggest that most of the SCG/L/R genes are expressed, with distinct patterns during the infectious cycle. The six SCGR subfamily genes are clustered and interspersed with the two SCA genes responsible for developmentally regulated arabinosylation of PG scGals; relationships amongst the SCGR revealed clear evidence of extensive gene conversion. In contrast, the seven SCG 'core' family members are localized adjacent to telomeres. These telomeres share varying amounts of sequence upstream and/or downstream of the SCG ORFs, again providing evidence of past gene conversions. Multiple SCG1-7 RNAs were expressed simultaneously within parasite populations. Potentially, telomeric localization of SCG genes may function primarily to facilitate gene conversion and the elaboration of functional evolutionary diversity in the degree of PG sc-galactosylation observed in other strains of L. major.