The initiation codon affects ribosome binding and translational efficiency in Escherichia coli of cI mRNA with or without the 5' untranslated leader.

Translational efficiency of an AUG, CUG, GUG, or UUG initiation codon was measured for the naturally leaderless cI mRNA from bacteriophage lambda. In a cI-lacZ translational fusion, only AUG supported a high level of expression; GUG supported a low level of expression, while UUG and CUG expression was barely above background levels. Addition of an untranslated lac leader and Shine-Dalgarno sequence to cI increased expression but still showed a dependence on an AUG for maximum expression. cI-lacZ mRNA with an AUG initiation codon showed a greater in vitro ribosome binding strength and a higher level of full-length in vivo mRNA, suggesting that the initiation codon is an important determinant of ribosome binding strength and translational efficiency for mRNA with or without the 5' untranslated leader.