Final answer:
Transposons are regulated by histone modification, which involves changes in chromatin structure, and mRNA degradation pathways, which include mechanisms using noncoding RNAs like miRNAs and siRNAs.
Step-by-step explanation:
Transposons are regulated by several mechanisms excluding PIWI-piRNAs. Among these, histone modification is a key method of controlling their activity. Histone modifications can lead to the formation of heterochromatin, which silences transposon expression by making the DNA less accessible to the cellular machinery that would otherwise initiate transcription. Adding methyl groups to histones (a process known as histone methylation) is one common way that cells can modify chromatin structure to suppress transposons.
Another method for regulating transposons is through mRNA degradation pathways, such as those involving microRNAs (miRNAs) and small interfering RNAs (siRNAs). These small noncoding RNAs can target transposon mRNA and lead to its destruction, thus preventing the transposon from mobilizing.
While DNA repair mechanisms and telomere extension are important cellular processes, they are not directly involved in the regulation of transposons as specified in the question.