I am trying to create a library of promoters to test them on my enzymes. I bought two kits from addgene which contained the promoters. However, for the same Promoter, i realized one has the kozak sequence and one does not. Is there a reason? For the one that does not contain the kozak sequence, I also noticed that the ORF right after it after their cloning also does not start with the kozak sequence. Is kozak sequence that important? How should I move on?
You need some kind of ribosome binding site (RBS), in order to get translation. Kozak's consensus sequence is one. It is very tolerant to changes. You can't necessarily tell by eye whether any given sequence is a functional RBS. The positioning of the RBS relative to the start codon is very important and intolerant to changes, but positioning of the promoter is not.
So you can have a vector, where for example you have a long multiple cloning site that you don't want translated. In those cases, it would make sense to include a promoter, but not a RBS, in the vector. It would be left to the user to place their RBS. Most genes come with one anyway.
Note that a promoter is relevant on the DNA level for the efficient initiation of transcription and the Kozak sequence on the mRNA for the efficiency of translation initiation. Make sure not to compare apples and oranges and keep one of these constant while studying different variants of the other.
A Kozak sequence is found at the translation start site (the methionine ATG) in the mRNA, which can be hundreds of nucleotides away from the promoter or the 5' transcription start site. The rest of your question does not make a lot of sense.
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