<italic>
<named-content content-type="subspecies">
<ext-link xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="uri" xlink:href="http://doi.org/10.1601/nm.14878" xlink:type="simple">Mycobacterium avium</ext-link>
</named-content>
</italic> subspecies <italic>
<named-content content-type="subspecies">
<ext-link xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="uri" xlink:href="http://doi.org/10.1601/nm.14878" xlink:type="simple">hominissuis</ext-link>
</named-content>
</italic> (MAH) is a pathogen that causes various non-tuberculous mycobacterial diseases in humans and animals worldwide. Among the genus, MAH is characterized by relatively slow growth. Here, we isolated a rapidly growing variant of the MAH 104 strain. The variant strain (named N104) exhibited an enhanced growth rate and higher motility compared to the parent MAH 104 strain (P104). Whole-genome sequencing analysis of N104 revealed the loss of the stop codon of <italic>MAV_RS14660</italic> due to a single nucleotide replacement, resulting in the substitution of the codon for tryptophan. Notably, exclusion of the stop codon ligated the open reading frames and caused the fusion of two adjacent proteins. A revertant parent strain, in which a mutation was introduced to restore the stop codon, revealed that elimination of the stop codon in <italic>MAV_RS14660</italic> was responsible for the N104 phenotype. Furthermore, we analysed the phenotypes of the parent and mutated strains by determining the functions of the <italic>MAV_RS14660</italic> and <italic>MAV_RS14655</italic> coding regions flanking the stop codon. The mutant strains, expected to express a fusion protein, exhibited increased resistance to antimicrobial drugs and exogenous copper toxicity compared to that of the parent strains. These findings suggest that the fusion of the <italic>MAV_RS14660</italic>- and <italic>MAV_RS14655</italic>-encoding regions in the mutant N104 strain could be related to the modified functions of these intrinsic proteins.