The protozoan parasite Giardia lamblia synthesizes a diverse and surprising
ly abundant array of sterile transcripts unable to code for proteins. Rando
m sampling of cDNAs from two evolutionarily divergent Giardia strains indic
ates that similar to 20% of cDNAs in the libraries represent polyadenylated
sterile transcripts. RNase protection analysis and northern blot hybridiza
tion of three sterile transcript loci demonstrated that both the sterile tr
anscript and a complementary mRNA were made in each case, further categoriz
ing these sterile transcripts as antisense transcripts. Investigation of th
e genomic loci for these same three sterile antisense transcripts showed ty
pical transcription units for the sense transcripts, but still failed to re
veal a usable open reading frame for the sterile antisense transcripts. 5'-
RACE mapped the transcription start site for one of the sterile antisense t
ranscripts to an AT-rich region, as is typical for Giardia. It is unclear w
hether these sterile transcripts represent errors in transcription or wheth
er they have regulatory functions within the cell, although preliminary inv
estigations failed to reveal evidence for a role in developmental gene regu
lation. In either case, the presence of such a large pool of sterile antise
nse transcripts is dramatic evidence of the unusual molecular machinery of
the early diverging protist G. Iamblia.