How can rare codon problems be reduced in a protein expression host?

Correct answer: By codon optimization or a suitable strain

A rare codon is a codon used infrequently by the chosen host, often because the corresponding tRNA is scarce. During translation of a heterologous gene, repeated rare codons can slow ribosome movement, cause translational pausing, reduce yield, or increase misfolding. Option A is correct because synonymous codon optimization can redesign the coding sequence for the host without changing the amino-acid sequence, while a suitable expression strain may supply additional tRNAs. Heating agar, removing selection, or drying the host does not solve a codon–tRNA mismatch and can damage cells or invalidate the culture.