Can the header line contain arbitrary text in FASTA format?

Question

The following is a valid .fasta file content:

>HSBGPG Human gene for bone gla protein (BGP) GGCAGATTCCCCCTAGACCCGCCCGCACCATGGTCAGGCATGCCCCTCCTCATCGCTGGGCACAGCCCAGAGGGT ATAAACAGTGCTGGAGGCTGGCGGGGCAGGCCAGCTGAGTCCTGAGCAGCAGCCCAGCGCAGCCACCGAGACACC ATGAG.

Is this also?

>Arbirary_Name_iJustCameUP_with_and_other_local_identifiers GGCAGATTCCCCCTAGACCCGCCCGCACCATGGTCAGGCATGCCCCTCCTCATCGCTGGGCACAGCCCAGAGGGT ATAAACAGTGCTGGAGGCTGGCGGGGCAGGCCAGCTGAGTCCTGAGCAGCAGCCCAGCGCAGCCACCGAGACACC ATGAG.

Answer 1

NCBI's FASTA format description:

https://www.ncbi.nlm.nih.gov/genbank/fastaformat/

NCBI's BLAST page describing valid FASTA input:

https://blast.ncbi.nlm.nih.gov/Blast.cgi?CMD=Web&PAGE_TYPE=BlastDocs&DOC_TYPE=BlastHelp

NCBI's SNP page describing FASTA format:

https://www.ncbi.nlm.nih.gov/projects/SNP/snp_legend.cgi?legend=fasta

UniProt's FASTA-header description:

https://www.uniprot.org/help/fasta-headers

Wikipedia reference (indicating format origin):

https://en.wikipedia.org/wiki/FASTA_format

FASTA program (origin of format, per Wikipedia):

https://fasta.bioch.virginia.edu/wrp_fasta/fasta_guide.pdf

Harvard PolyPhen page describing FASTA format:

http://genetics.bwh.harvard.edu/pph/FASTA.html

FASTA file format (from Pacific Biosciences):

https://pacbiofileformats.readthedocs.io/en/3.0/FASTA.html

Answer 2

I am not aware of an official fasta format description. The only constraint that I know of is that there should not be any whitespace after >. Apart from that particular position, whitespaces are allowed in the header line.

So, regarding your particular example, both are valid fasta sequences. When in doubt, you can use SeqIO from Biopython, if you can parse your file with the following code, it is should be a valid fasta file.

from Bio import SeqIO

with open("example.fasta") as handle:
    for record in SeqIO.parse(handle, "fasta"):
        print(record.id)

Edit per @Chris_Rands' comment

The code below does the same as above, meaning that SeqIO.parse() takes care of opening and closing the file.

from Bio import SeqIO

for record in SeqIO.parse("example.fasta", "fasta"):
    print(record.id)