Free minicircles of kinetoplast DNA in Crithidia fasciculata

The major form of kinetoplast DNA in Crithidia fasciculata is a network which contains thousands of minicircles linked together in a two-dimensional array. This paper reports the existence of free minicircles in Crithidia which by several criteria are identical to those in networks. They are the same size (about 2500 base pairs), and they yield the same products upon digestion with restriction enzymes. About 0.4% of the minicircles in exponentially growing nonsynchronized cells are free and the remainder are in networks. After a 5-min pulse with [3H]thymidine, above 10% of all of the incorporated radioactivity in the cell is in free minicircles, and the minicircles have a higher specific radioactivity than the average of other DNAs in the cell. Three-branched structures, which resemble Cairns-type replication intermediates, are occasionally observed by electron microscopy. Kinetic studies of the incorporation of [3H]thymidine into free minicircles indicate that they turn over, and this turnover was confirmed by a pulse-chase experiment. These properties of free minicircles suggest that they may be intermediates in the replication of network minicircles.