To examine the degree of sequence similarities at an amino acid level between each query protein and the T4SEfinder-collected T4SEs, the NCBI BLASTp-derived Ha-value was employed. For each query, the Ha-value was calculated as follows: where i was the level of BLASTp identities of the region with the highest Bit score expressed as a frequency of between 0 and 1, l_m the length of the highest scoring matching sequence (including gaps) and l_q the query length. If there were no matching sequences with a BLASTp E-value < 0.01, the Ha-value assigned to that query sequence was defined as zero. Therefore Ha-value belonged to the set, Ha ∈[0,1]. Here, a strict Ha-value cut-off ≥ 0.42 was used to determine the significant sequence similarities.