​The mRNA of the E. coli lac operon contains the open reading frames for lacZ, lacY, and lacA genes from a single cistron. It is observed that lacZ is translated more frequently than lacY or lacA. Which one of the following statements best describes the reason for this observation?​

Correct option is C

Explanation:

The differential translation frequency of lacZ, lacY, and lacA is mainly due to differences in the efficiency of translation initiation.

• The Shine-Dalgarno (SD) sequence is a ribosome binding site upstream of each gene's start codon.

• Variations in the SD sequence strength for each gene affect ribosome affinity and thus translation initiation rates.

• lacZ has a stronger SD sequence compared to lacY and lacA, leading to more frequent translation initiation.

• The SD sequences are present upstream of all three genes, ruling out option 1.

• Inhibitory RNA structures can influence initiation but are not the primary reason here, ruling out options 2 and 4.

Hence, option 3 is correct.

Information Booster:

• The lac operon is polycistronic; however, the translation efficiency of each gene can vary due to differences in SD sequences.

• Stronger SD sequences recruit ribosomes more effectively, resulting in higher translation frequency.

• This mechanism allows differential protein production from the same mRNA transcript.