Consistent association between sigma elements and tRNA genes in yeast.

Sigma is a recently described family of transposable elements in yeast (Saccharomyces cerevisiae). The most striking feature of the seven sigma elements that have been previously identified is that all are located 16-18 base pairs upstream from tRNA-encoding regions. Because these cases were all encountered in the process of studying specific tRNA genes, the full extent of the association between sigma elements and tRNA genes could not be assessed. In this paper, we report a more global characterization of the sigma family in a typical laboratory yeast strain: of the 30 copies of sigma that we estimate to be present in the haploid genome, we have cloned and analyzed 25 loci. Although in two cases a pair of sigma elements were found within several kilobases of each other, the majority occur as individual elements at widely dispersed sites. Moreover, in all 25 cases analyzed, the sigma elements are closely associated with tRNA genes. Thus, the sigma transposable element has been shown to have an absolute association with another gene family.