MOLECULAR CHARACTERIZATION OF THE LARGEST SUBUNIT OF PLASMODIUM-FALCIPARUM RNA POLYMERASE-I

Plasmodium species possess developmentally regulated ribosomal RNA (rR NA) genes. This report describes the expression and gene structure of the largest subunit of P. falciparum RNA polymerase I (RNAPI), which i s responsible for the synthesis of rRNA. The RNAPI largest subunit gen e was present as a single copy gene on chromosome 9. Three exons encod e the 2910-amino acid RNAPI polypeptide (340 140 Da). A comparison of Plasmodium, Trypanosoma, and Saccharomyces cerevisiae nuclear RNAP lar gest subunits identified conserved amino acid positions and class-spec ific amino acid positions. Novel amino acid insertions were found betw een RNAPI conserved regions A and B (region A'), D and DE1 (region D') , DE2 and E (region DE2'), and F and G (region F'). Leucine zipper dom ains were found within regions D', DE2, and DE2'. A novel serine-rich repeat domain, a domain with homology to the C-terminal domain of euka ryotic upstream binding factor (UBF), and 4 highly conserved casein ki nase II (CKII) Ser/Thr phosphorylation motifs were found within a 127- amino acid subregion of enlarged region F'. The novel RNAPI serine-ric h repeat contained a conserved motif, Ser-X3-Ser, which was also ident ified in the serine-rich repeat domains of the P.falciparum RNAPII and RNAPIII largest subunits, as well as within a highly homologous serin e-rich repeat from trophozoite antigen R45. The results of this molecu lar analysis indicate that phosphorylation and dephosphorylation mecha nisms regulate the activity of P. falciparum RNAPI.