Instability of a yellow mutation in Chlamydomonas reinhardtii is not due to TOC1 elements

yc4 is an unstable yellow-in-the dark mutation found in the unicellular green alga Chlamydomonas reinhardtii. yc4 reverts to the wild-type green-in-the-dark phenotype at a frequency of 1 X 10(-4) revertants per cell division. We have tested whether mutation and reversion of yc4 is due to the insertion and excision of the C. reinhardtii transposon TOC1. Southern blots showed differences in the distribution of TOC1 elements when the original mutant yc4 strain, which contained over 30 TOC1 elements, was compared with green-in-the-dark-revertants. However, analysis of the yc4 mutation in a different genetic background containing half the number of TOC1 elements showed that reversion of yc4 is not associated with loss of TOC1 elements.