AN AUG INITIATION CODON, NOT CODON-ANTICODON COMPLEMENTARITY, IS REQUIRED FOR THE TRANSLATION OF UNLEADERED MESSENGER-RNA IN ESCHERICHIA-COLI

We determined the in vivo translational efficiency of 'unleadered' lac Z compared with a conventionally leadered lacZ with and without a Shin e-Dalgarno (SD) sequence in Escherichia coli and found that changing t he SD sequence of leadered lacZ from the consensus 5'-AGGA-3' to 5'-UU UU-3' results in a 15-fold reduction in translational efficiency; howe ver, removing the leader altogether results in only a twofold reductio n. An increase in translation coincident with the removal of the leade r lacking a SD sequence suggests the existence of stronger or novel tr anslational signals within the coding sequence in the absence of the l eader. We examined, therefore, a change in the translational signals p rovided by altering the AUG initiation codon to other naturally occurr ing initiation codons (GUG, UUG, CUG) in the presence and absence of a leader and find that mRNAs lacking leader sequences are dependent upo n an AUG initiation codon, whereas leadered mRNAs are not. This sugges ts that mRNAs lacking leader sequences are either more dependent on pe rfect codon-anticodon complementarity or require an AUG initiation cod on in a sequence-specific manner to form productive initiation complex es. A mutant initiator tRNA with compensating anticodon mutations rest ored expression of leadered, but not unleadered, mRNAs with UAG start codons, indicating that codon-anticodon complementarity was insufficie nt for the translation of mRNA lacking leader sequences. These data su ggest that a cognate AUG initiation codon specifically serves as a str onger and different translational signal in the absence of an untransl ated leader.