StringPadLeft

makes string be of length n, padding it on the left with spaces or truncating it if necessary.

StringPadLeft["string",n,"padding"]

pads by repeating copies of the string padding.

StringPadLeft[{"s₁","s₂",…}]

pads strings with spaces on the left to make them all the same length.

StringPadLeft[{"s₁","s₂",…},n,…]

pads or truncates to make all strings of length n.

Details

StringPadLeft effectively places repeated copies of padding to the left of string, truncating them when a total width of n characters has been achieved.
StringPadLeft[BioSequence["type","seq"],n,…] will apply the operation to the string "seq", yielding a BioSequence result.
The following letter is used as the default padding for each type of biomolecular sequence if not otherwise specified:
"DNA" or "CircularDNA" N

"RNA" N

"Peptide" X

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Pad a string with whitespace to have total length 10:

Pad with a non-whitespace character:

Make a list of strings the same length:

Pad or truncate a list of strings to a given length:

Pad a DNA sequence with characters representing an unknown upstream sequence:

Pad a DNA sequence with a particular letter:

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