Te. Nash et al., GIARDIA-LAMBLIA - IDENTIFICATION AND CHARACTERIZATION OF A VARIANT-SPECIFIC SURFACE PROTEIN GENE FAMILY, The Journal of eukaryotic microbiology, 42(5), 1995, pp. 604-609
Giardia lamblia trophozoites undergo antigenic variation by modulating
the expression of variant-specific surface proteins (VSP), which are
encoded by a number of small multigene families. We characterized the
genomic copy of the VSP gene expressed by the cloned trophozoite line
H7, derived from the isolate GS/M, in addition to a related, but nonex
pressed, family member. The coding regions of the two genes encode clo
sely related polypeptides (86% identity). However, differences in the
coding region of these genes reside solely in an 873 bp segment. Only
four differences were found between the 5' flanking sequences (465 bp)
. The proximal 205 base pairs downstream from the coding regions were
identical, but thereafter the sequences diverged (37% identity over th
e next 391 bp). Mapping studies indicated that no other VSP gene was l
ocated within 4 kb pairs of the expressed H7 VSP gene, and transcripts
from the nonexpressed gene were detected in neither GS/H7 nor heterog
eneous trophozoites populations derived from this cloned line. Any mec
hanisms responsible for the differential expression of VSP genes must
reconcile the near identity of DNA sequences that flank the coding reg
ions of expressed and nonexpressed VSP genes.