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Euglena LHCPII polyprotein precursor: Unique gene structure and processing pathway
Abstract
The Euglena light-harvesting chlorophyll a/b binding proteins of PSII (LHCPIIs) are synthesized on membrane bound cytoplasmic ribosomes as very large polyprotein precursors (pLHCPIIs), encoded by 6.6 and 9.5 kb transcripts. Sequence analysis of two genomic clones, GC18 and GC7, encoding the 3$\sp\prime$ and 5$\sp\prime$ portions of the Euglena LHCPII gene(s), respectively, indicates that pLHCPII consists of a 141 amino acid N-terminal extension and 8-9 mature LHCPIIs covalently joined together by decapeptide linkers. Three types of LHCPII are contained within the polyprotein precursor and their overall structure is similar to that of other characterized LHCPIIs. The Euglena genome appears to contain 3-5 LHCPII polyprotein genes and at least 2 of them are transcribed into 6.6 kb mRNAs. Several introns are present in the LHCPII genes as identified through comparison between the genomic sequence and the corresponding cDNA sequences. The fourteen identified Euglena introns do not contain the highly conserved GT and AG dinucleotides at their ends, as found for almost all other nuclear pre-mRNA introns characterized to date from various eukaryotic organisms. The Euglena introns contain a purine and a pyrimidine at their 5$\sp\prime$ and 3$\sp\prime$ ends, respectively. The ends of Euglena introns contain a short stretch of nucleotides which can base-pair to bring the two ends together for splicing. A 26 nucleotide sequence, present in the mRNA sequence but absent from the adjacent genomic sequence, appears to be added to the 5$\sp\prime$ end of Euglena LHPCII pre-mRNA by trans-splicing. The Euglena pLHCPII N-terminal extension contains three hydrophobic domains and a potential signal peptidase cleavage site at amino acid 35. Cotranslational processing by canine microsomes removed approximately 3 kDa from an in vitro synthesized truncated pLHCPII. The second hydrophobic domain of the presequence may serve as a stop transfer sequence, anchoring pLHCPII into the endoplasmic reticulum membrane, thereby routing it through the Golgi prior to chloroplast localization. The pathway for synthesis and processing of nuclear encoded chloroplast proteins, and the organization and processing of genetic information is clearly unique in Euglena.
Subject Area
Molecular biology|Cellular biology
Recommended Citation
Muchhal, Umesh Shyamsunder, "Euglena LHCPII polyprotein precursor: Unique gene structure and processing pathway" (1993). ETD collection for University of Nebraska-Lincoln. AAI9415985.
https://digitalcommons.unl.edu/dissertations/AAI9415985