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DNA 序列的 GC 含量是指序列中'G'和'C'所占的百分比。

一条 DNA 序列很容易表示，但是如果有多条 DNA 序列放在一起，则每条序列必须被标记，通常的做法是保存为 FASTA 格式文件。在这种格式中，序列的名称占一行，名称的最前面是一个大于符号‘>’开头，序列名称后面可以跟一系列说明；序列信息从名称的下一行开始，直到遇到下一个以‘>’开头的序列名称为止。Fasta 格式文件可参考下面的示例数据。

给定：一个 Fasta 序列文件。

需得：GC 含量最高的序列名称及其 GC 含量（各占一行行输出）。

示例数据

>Rosalind_6404
CCTGCGGAAGATCGGCACTAGAATAGCCAGAACCGTTTCTCTGAGGCTTCCGGCCTTCCC
TCCCACTAATAATTCTGAGG
>Rosalind_5959
CCATCGGTAGCGCATCCTTAGTCCAATTAAGTCCCTATCCAGGCGCTCCGCCGAAGGTCT
ATATCCATTTGTCAGCAGACACGC
>Rosalind_0808
CCACCCTCGTGGTATGGCTAGGCATTCAGGAACCGGAGAACGCTTCAGACCAGCCCGGAC
TGGGAACCTGCGGGCAGTAGGTGGAAT

示例结果

Rosalind_0808
60.919540

Python 实现

Computing_GC_Content.py

import sys
import pysam

def gc_content(item):
    n, s = item
    return (s.count('G') + s.count('C')) * 100 / len(s)

def max_gc_content(infasta):
    dna = {}
    with pysam.FastxFile(infasta) as fh:
        for r in fh:
            dna[r.name] = r.sequence.upper()
    return max(dna.items(), key=gc_content)

def test():
    item = max_gc_content('rosalind_gc_test.txt')
    return item[0] == 'Rosalind_0808' and round(gc_content(item), 6)== 60.919540

if __name__ == '__main__':
    if not test():
        print("cout_gc_content:Failed")
        sys.exit(1)

    item = max_gc_content('rosalind_gc.txt')
    print(item[0])
    print(gc_content(item))

本题要点：

1. 用 pysam 读取 Fasta 文件，并将其放入字典中；详细用法见：基因组文件读写（pysam）
2. max 函数的使用，特别是为其构造一个 key 函数并传入，这是解本题的关键，GC 含量本身是很容易理解的。

Problem

The GC-content of a DNA string is given by the percentage of symbols in the string that are 'C' or 'G'. For example, the GC-content of "AGCTATAG" is 37.5%. Note that the reverse complement of any DNA string has the same GC-content.

DNA strings must be labeled when they are consolidated into a database. A commonly used method of string labeling is called FASTA format. In this format, the string is introduced by a line that begins with '>', followed by some labeling information. Subsequent lines contain the string itself; the first line to begin with '>' indicates the label of the next string.

In Rosalind's implementation, a string in FASTA format will be labeled by the ID "Rosalind_xxxx", where "xxxx" denotes a four-digit code between 0000 and 9999.

Given: At most 10 DNA strings in FASTA format (of length at most 1 kbp each).

Return: The ID of the string having the highest GC-content, followed by the GC-content of that string. Rosalind allows for a default error of 0.001 in all decimal answers unless otherwise stated; please see the note on absolute error below.

Sample Dataset

>Rosalind_6404
CCTGCGGAAGATCGGCACTAGAATAGCCAGAACCGTTTCTCTGAGGCTTCCGGCCTTCCC
TCCCACTAATAATTCTGAGG
>Rosalind_5959
CCATCGGTAGCGCATCCTTAGTCCAATTAAGTCCCTATCCAGGCGCTCCGCCGAAGGTCT
ATATCCATTTGTCAGCAGACACGC
>Rosalind_0808
CCACCCTCGTGGTATGGCTAGGCATTCAGGAACCGGAGAACGCTTCAGACCAGCCCGGAC
TGGGAACCTGCGGGCAGTAGGTGGAAT

Sample Output

Rosalind_0808
60.919540