Different conformations of the 3' termini of initiator and elongator transfer ribonucleic acids. An EPR study

The 3' ends of transfer ribonucleic acids were covalently labeled with a nitroxide spin label. The 3' end of initiator tRNA (tRNAMetf) from Escherichia coli shows different motional behavior than the 3' terminus of elongator tRNAs as monitored by EPR. The line shapes of the EPR spectra are quite sensitive to the buffer conditions, as shown by measurements in 4 different buffers. The data are consistent with a constrained or folded back 3' terminus in the initiator tRNA as opposed to the freely rotating elongator 3' terminus. The EPR spectra are also sensitive to aggregation of the tRNA.