Molecular cloning of the A2 locus

The A2 locus is involved in anthocyanin synthesis and harbors a number of mutable alleles (Peterson, P. A., in Maize Breeding and Genetics, Wiley Interscience, Walden, D.B., ed., 1975) controlled by the En (Spm) system of transposable elements.

To enable study of molecular events at the A2 locus, a transposon tagging experiment was undertaken using En-specific probes to screen a genomic library constructed with a line containing the a2-m1(1511) allele, in which an autonomous En element is present at the A2 locus (Peterson, P. A., Genetics 59:391-398, 1968). One clone was isolated, lm305, which contained a complete 8.4 kb En element (Pereira, A., et al., EMBO J. 4:17-23, 1985) as shown by restriction mapping and heteroduplex analysis. Since it has been shown that the autonomous En elements from the A1 (O'Reilly, C. et al., EMBO J. 4:877-882, 1985), C2 (Wienand, U., et al., this volume), and C (Paz-Ares, J., et al., this volume) loci of maize are identical in structure by these criteria, the data suggested that sequences flanking the element in lm305 represent at least a portion of the A2 locus. An lm305-derived fragment was used to probe Southern blots of genomic DNA from lines carrying various alleles of the A2 gene. The results obtained are in good agreement with those expected for an A2-specific probe.

In order to directly analyze En-mediated gene suppression without interference from excision events, efforts are currently underway to isolate genomic clones for the class II state of a2-m1 (McClintock, B., Carnegie Inst. Wash. Yearbook 57:415-429, 1958), the structure and properties of which will hopefully further develop concepts concerning the mechanism (Schwarz-Sommer, Zs., et al., EMBO J. 4:2439-2443, 1985) of En (Spm) action in the maize genome.

Bill Martin, Alfons Gierl, Peter A. Peterson1 and Heinz Saedler

1Dept. of Agronomy, Iowa State Universitv

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