The Kozak sequence is important for efficient translation initiation and plays a role in determining the rate of translation initiation and the efficiency of protein synthesis. The canonical Kozak sequence, 5'-gccRccAUGG-3', is optimal for translation initiation because it provides the necessary information for the ribosome to correctly identify the start codon (AUG) and efficiently initiate translation. However, it is not always possible to have a strong Kozak sequence, especially when the second amino acid of the protein is not one of the five amino acids that can be coded for by codons that start with G.

In cases where it is not possible to have a second codon that starts with G, there may be alternative ways to optimize the Kozak sequence. For example, some studies have suggested that the presence of certain nucleotides at positions -3 and +5 can enhance the efficiency of translation initiation even in the absence of a G at position +4. Additionally, the sequence context surrounding the start codon can also affect the efficiency of translation initiation. Therefore, it is possible to have a strong Kozak sequence even if the second amino acid of the protein is not one of the five amino acids that can be coded for by codons that start with G.

In summary, while the presence of a G at position +4 in the Kozak sequence is optimal for translation initiation, it is not always possible, especially when the second amino acid of the protein is not one of the five amino acids that can be coded for by codons that start with G. In such cases, alternative sequence features and context may compensate for the absence of a G at position +4 and still allow for efficient translation initiation.