Вы можете сделать что-то подобное в Awk:
awk '
NR==FNR {a[">"$1] = ">"$0; next} $1 in a {$1 = a[$1]} 1
' df2.txt df1.txt
Пр.
$ awk 'NR==FNR {a[">"$1] = ">"$0; next} $1 in a {$1 = a[$1]} 1' df2.txt df1.txt
>tr|A0A1B1L9R9|A0A1B1L9R9_BACTU ABC transporter permease OS=Bacillus thuringiensis OX=1428 GN=berB PE=4 SV=1
MNKQLFLASLKETQKSILSYACGAALYLWLLIWIFPSMVSAKGLNELIAAMPDSVKKIVG
MESPIQNVMDFLAGEYYSLLFIIILTIFCVTVATHLIARHVDKGAMAYLLATPVSRVQIA
ITQATVLILGLLIIVSVTYVAGLVGAEWFLQDNNLNKELFLKINIVGGLIFLVVSAYSFF
FSCICNDERKALSYSASLTILFFVLDMVGKLSDKLEWMKNLSLFTLFRPKEIAEGAYNIW
PVSIGLIAGALCIFIVAIVVFKKRDLPL
>sp|O15304|SIVA_HUMAN Apoptosis regulatory protein Siva OS=Homo sapiens OX=9606 GN=SIVA1 PE=1 SV=2
MPKRSCPFADVAPLQLKVRVSQRELSRGVCAERYSQEVFEKTKRLLFLGAQAYLDHVWDE
GCAVVHLPESPKPGPTGAPRAARGQMLIGPDGRLIRSLGQASEADPSGVASIACSSCVRA
VDGKAVCGQCERALCGQCVRTCWGCGSVACTLCGLVDCSDMYEKVLCTSCAMFET