Abstract

rRNA genes of Entamoeba histolytica are organized as palindromic ribosomal DNA (rDNA) units (I and II) in a 24.5-kb circle. Although the two rDNAs are identical in sequence, their upstream spacers are completely different. Since the intergenic sequences (IGS) of all rDNA copies in other organisms are conserved and contain transcription regulatory sequences, the lack of sequence conservation in the IGS prompted the question of whether both rDNAs are indeed transcriptionally active. We mapped the transcriptional start points (tsp's) and promoters of the two rDNAs. A 51-bp sequence immediately upstream of the tsp's was highly conserved in both units. In addition, both units had an A+T-rich stretch upstream of the 51-bp core. Analysis of reporter gene transcription showed promoter activity to reside in the regions from positions -86 to +123 (rDNA I) and positions -101 to +140 (rDNA II). The promoter-containing fragments from both units could bind and compete with each other for protein(s) from nuclear extracts. Protein binding was especially dependent on the A+T-rich region upstream of the 51-bp core (positions -53 to -68). The requirement of >80 bp downstream of the tsp was striking. Although this sequence was not conserved in the two units, it could potentially fold into very long stem-loops. Both rDNAs transcribed with comparable efficiency, as measured by nuclear runon. Thus, both rDNAs share very similar organization of promoter sequences, and in exponential culture both rDNAs are transcribed. It remains to be seen whether the different IGS affect the regulation of the two units under adverse conditions.

Talk to us

Join us for a 30 min session where you can share your feedback and ask us any queries you have

Schedule a call

Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.