Abortive translation caused by peptidyl-tRNA drop-off at NGG codons in the early coding region of mRNA

In Escherichia coli the codons CGG, AGG, UGG or GGG (NGG codons) but not GGN or GNG (where N is non-G) are associated with low expression of a reporter gene, if located at positions +2 to +5. Induction of a lacZ reporter gene with any one of the NGG codons at position +2 to +5 does not influence growth of a normal strain, but growth of a strain with a defective peptidyl-tRNA hydrolase (Pth) enzyme is inhibited. The same codons, if placed at position +7, did not give this effect. Other codons, such as CGU and AGA, at location +2 to +5, did not give any growth inhibition of either the wild-type or the mutant strain. The inhibitory effect on the pth mutant strain by NGG codons at location +5 was suppressed by overexpression of the Pth enzyme from a plasmid. However, the overexpression of cognate tRNAs for AGG or GGG did not rescue from the growth inhibition associated with these codons early in the induced model gene. The data suggest that the NGG codons trigger peptidyl-tRNA drop-off if located at early coding positions in mRNA, thereby strongly reducing gene expression. This does not happen if these codons are located further down in the mRNA at position +7, or later.