Maize Genetics Cooperation Newsletter vol 86 2012

On Ac induced chromosome dissociation

Barbara McClintock was able to identify the presence of the activator, Ac, by its action

of chromosome dissociation, Ds. Although the events involved in the mobility the Ds

transposon are clear, I know of no studies on Ac breakage of the chromosome. Three

possible events come to mind. Breakage can result from Ds mobility if the two broken

ends do not join to produce an eight base direct repeat (1). It may result from a single

break at the 5� end that contains the nearby AAACGG binding site of the transposase (2).

The last event (3) is breakage at the 3� end. The fusion of the two complimentary strands

of DNA at a break point that must occur to give rise to McClintock�s breakage–fusion-

bridge cycle, affords a simple way to distinguish between these three possible events

Ds1 terminal sequences of about 40 bases were duplicated and pasted end to end,

either at the 5� or at the 3� termini, and used as query in BLASTs of the sequenced

genome of B73. The BLASTs were searched for hits that extended across the paste

point. According to (1), no such hits would be expected when either the duplicate

5�-5� or 3�-3� terminal sequences were used as query, since the fusion would join the

flank sequences and not Ds1. According to (2) hits that extend across the paste point

should be detected only with the duplicate 5-5� used as query and not with the

duplicate 3�-3� query. The opposite should result according to event (3) with the

extended hits detected only when the query is the duplicated 3�3� sequence.

Tests with a number of Ds transposons clearly showed that event (2) is correct.

In BLASTs with duplicate 5�-5� pasted termini as query, multiple cases were detected

with hits that extended for as long as 20 bases on both sides of the paste point. No such

cases were detected with the duplicated 3�-3� pasted queries.

The AAACGG transposase binding site is very close to the terminus, starting at position

9. Proximity is not required for the cleavage that occurs at both the termini in the case of

Ac/Ds transposon mobility where the 5� and 3� termini are separated by as many as 4500

bases, but it may be a factor in breakage at only one end that gives rise to

a breakage-fusion-bridge cycle. In order to check if a close by transposase binding site at

the 3�end would give a positive result, a test was performed using Ds transposons with

terminal sequences that had an AAAGGG sequence close to both the 5� and 3� ends. The

test results were clearly positive showing that the breakage could occur at either the 5� or

3� termini.

Drew Schwartz

Hebrew University of Jerusalem

Please Note: Notes submitted to the Maize Genetics Cooperation Newsletter may be cited only with consent of authors.