Maize Genetics Cooperation Newsletter vol 86 2012

Activation of silent Ac sequences

In a comparison of active and silent Ac sequences I have found that a single nucleotide

polymorphism always appears at position 20, counting from the Ac9 5' end. All active Ac

sequences have a T at that position. The B73 sequenced genome does not contain an

active Ac, only a silent 4554 base Ac as well as 55 Ac fractionals that begin at the 5'end.

All these have a G at that position. The portions of Ac which are critical to the formation

of the transposase, promoter and the translated segment, extend from the 5' end to base

2451 of Ac9. The first 3100 bases of the active Ac9 and the 4554 base Ac, are identical

except for seven single nucleotide polymorphisms. Of these, only the T to G change at

base 20, is seen in all the 56 Ac sequences in the B73 genome. A T to G change is a rare

mutation, but it is always seen at base 20 in silent Ac. By comparison, of the transposons

with strong similarity to Ac that is limited to the first 110 bases, only one out of 175 have

G in place of T at position 20. Also, of the 17 transposons in which the similarity to

Ac is limited to the first 206 bases, only one has a G in place of T at position 20.

Studies are currently being undertaken to test the hypothesis that stress induced

activation of the silent Ac involves a G to T nucleotide change at position 20. The

transposase activity of Ac9 causes much damage to maize plants, as a result of

chromosome breakage with the subsequent, breakage, bridge, fusion cycle, as well as

gene inactivation from Ac induced insertions. Thus, in a stable environment, plants with

an active Ac would be selected against. However, under conditions of stress caused by a

drastic changes in the environment, an active Ac would play a major role in producing

the mega-changes required for survival of the species and what would be better than

activation induced by the simple change of only a single nucleotide.

Drew Schwartz

Hebrew University of Jerusalem

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