CYTOSINE-73 IS A DISCRIMINATOR NUCLEOTIDE IN-VIVO FOR HISTIDYL-TRANSFER RNA IN ESCHERICHIA-COLI

The acceptor helix of histidine tRNAs in Escherichia coli is capped by a unique base pair in which the cytosine at the discriminator positio n is paired with an extra guanosine at -1. In previous in vitro studie s, the presence of the G-1:C73 base pair was found to be required to o btain both optimal histidylation by histidyl-tRNA synthetase and accur ate 5' processing by RNase P. We investigated the role of G-1:C 73 in histidine tRNA identity and found that nucleotide substitutions confer red mischarging by other amino acids in a pattern that correlated with the discriminator base and not with the extra nucleotide at -1. As sh own by primer extension experiments, the relatively minor role of the -1 nucleotide in vivo could be attributed to altered RNase P processin g. These studies show that interactions of tRNAs in vivo both with RNa se P during tRNA biosynthesis and with the pool of aminoacyl-tRNA synt hetases can modulate the effects of substitutions at recognition nucle otides, eliciting changes in transfer RNA identity.