вот

gistfile1.py

f = open("test_7.txt",'r')
count_GC = 0
str_ = []
while 1:
    str1 = f.readline()
    if str1 == "":
        print str_[-1],count_GC
        break
    str2 = f.readline()
    temp1 = str2
    if str2 == "": break
    while temp1[0]!='>':
	    temp1 = f.readline()
	    str2 = str2 + temp1
        if temp1 == "": 
		    temp1 = '>'
    print str1,str2
    count_next = float(str2.count("G")+str2.count("C"))/float(len(str2))
    if count_GC < count_next:
        count_GC = count_next
        str_.append(str1)