Closed:Finding if Sequence is Present in Long List of Other Sequences from Multiple Species

Question

[gtasource]

Hello,

I have a sequence of 500nt, that has an important function in Species X. I have a list of about 20 other species, with coordinates of about ~20,000, that I want to see if they contain this important 500nt region. I did an initial BLASTN between this sequence and the 20 other sequences, and I haven't seen anything significant. I went and did a BLAST for "Somewhat similar sequences", and I'm getting hits, but they're very short, and some have not the greatest E values.

What are some ways to make my search more thorough so I can more confidently state that these 20 species do not contain this important 500nt sequence.