The abundance of sterile transcripts in Giardia lamblia

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.