def find_seq_sources(dfs):
    """
    Takes DataFrame list of DataFrames containing biosamples

    Returns dictionary {database1:[SAMN01, SAMN02],
                        database2:[SAMN02, SAMN03],
                        database3:[SAMN04]}
    """
    sources = {}

    for df_name in dfs.keys():
        if df_name.upper() == 'NCBI':
            sources[df_name] = dfs[df_name]['BioSample']
        elif df_name.upper() == 'PATRIC':
            sources[df_name] = dfs[df_name]['biosample_accession']
            #sources[df_name] = dfs[df_name]['BioSample']
        else:
            raise Exception("key not set for database {}".format(df_name))

    return sources