length-dependent cis-regulatory function of A-repeats and that Argonaute proteins serve as trans-acting factors, binding to A-repeats.
INTRODUCTION A microsatellite or a tandem repeat (TR) is a concatenation of the same nucleotide sequence, called a unit. In other words, a TR is a repeat of the same unit of nucleotides from the beginning to the end of the repeat (1,2). For example, ‘AAAAA’ represents five repeats of ‘A’, whereas ‘CATCATCATCAT’ represents four repeats of ‘CAT’. Traditionally, these repeats were believed to be generated by DNA replication slippage and to have no function, and they were called ‘junk’ or ‘selfish’ DNA (3). TRs have a propensity for evolvability because there is a high degree of variation within the TRs among related species (4). Repeat variation can be measured in terms of repeat size and sequence similarity (5). TRs are found ubiquitously in both coding and non-coding regions. In coding regions, TRs enable functional variability among genes. In non-coding regions, specifically within gene promoters, repeat variability correlates with variations in gene expression (4,6). This diversity of expression can produce phenotypic variants. Several lines of evidence showing phenotypic variations due to TRs have been reviewed (2). The evolvability of gene modulation is vital for coping with environmental changes and for the emergence of new species. TRs in promoters mediate transcription in