Suppressors of a UGG missense mutation in Escherichia coli

As part of our investigation of tRNA structure-function relationships, we isolated and preliminarily characterized translational suppressors of the tryptophan codon UGG in a trpA missense mutant of Escherichia coli. the parent strain also contained two other mutant alleles relevant to the suppressor search; these were supD, which codes for a serine-inserting amber suppressor tRNA, and gly V55, the gene for a GGA/G-reading mutationally altered glycine tRNA. On the basis of map location, reversed-phase (RPC-5) column chromatography of glycyl-tRNA, and codon response, several classes have been distinguished so far. The number of suppressors in each class, their codon responses, and their apparent genic identities, respectively, are as follows: class 1--4 suppressors, UGG, supD; class 2--12 suppressors, UGG, glyU; class 3--9 suppressors, UGA and UGG, glyT; class 4--2 suppressors, UGG, glyT; class 5--7 suppressors, UGG, gly V55. Besides these, one suppressor retains supD activity, but so far its map location has not been distinguished from that of supD. Another suppressor clearly does not map near supD or any of the glycine tRNA genes mentioned. These last two suppressors may represent novel missense suppressors such as misacylated tRNA's or mutationally altered aminoacyl-tRNA synthetases, tRNA modification enzymes, or ribosomes. Finally, three other suppressors were obtained from a strain containing glyT56, the gene for an AGA/G-reading form of glyT tRNA. All three occurred at the expense of glyT56 activity and exhibited the the transductional linkage to argH that is characteristic of glyT.