def get_gene_symbol_of_transcript_id(transcripts_dict, transcript_id):
    """
    @description get the gene_symbol of a given transcript_id from the user gtf data
    @param transcripts_dict (dict)
    @param transcript_id (string)
    @return (None|string)
    """
    if transcript_id in transcripts_dict:
        t_list = transcripts_dict[transcript_id]
        if len(t_list) > 0:
            return t_list[0]['gene_symbol'].lower()
    return None
