Abstract
Primer extension has been employed to locate sites of cleavage made in apolipoprotein II (apo-II) mRNA by structure-specific nucleases. This approach permits structural analysis of specific mRNAs within a complex population. Electrophoretic analysis of cDNAs synthesized from T1 RNase-treated and mock-treated apo-II mRNA revealed that most cleavage sites can be mapped with single nucleotide accuracy. However, some T1 RNase-dependent cDNAs demonstrated mobilities corresponding to one nucleotide longer than the mRNA template, suggesting that reverse transcriptase can add a single nucleotide to full-length cDNAs in a template-independent reaction. This approach has been used to map double-stranded and single-stranded accessible domains of the 3' noncoding region of apo-II mRNA with cobra venom, T1, and S1 ribonucleases. Cleavage profiles of apo-II mRNA renatured under a variety of buffer and temperature conditions were identical and in no case was overlap observed between sites of cleavage by double strand- and single strand-specific enzymes. These results suggest that apo-II mRNA possesses a predominant, stable secondary structure. A computer-generated structure model, consistent with these nuclease cleavage data, is presented. In addition to the analysis of mRNA higher order structure in mixed RNA populations, this approach also appears suitable for the analysis of protein-mRNA interactions. Termination sites of incomplete cDNAs produced when untreated or mock-treated RNA is used as a template for primer extension were also mapped. This analysis revealed an over-representation of termination at the dinucleotides CA and CU, suggesting that termination of some incomplete apo-II cDNAs is related to primary and not secondary structure. Such sequence dependence could reflect in vivo degradation by an endogenous cytidine-specific nuclease.
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