The Hor 2 locus, that encodes the major B hordein group of seed storage proteins, has been estimated to consist of 20-30 gene copies (Bunce et al., 1986). Although two major groups (or subfamilies) of B hordein polypeptides can be recognized on the basis of their cyanogen bromide peptide maps (Faulks et al., 1981) and the cross-hybridization of their cDNAs (Kreis et al., 1983), recombination within the locus has not been detected. However, the presence of only single B hordein genes on genomic fragments of 17.4 and 4.7 kb (Forde et al., 1985); Brandt et al., 1985) indicates that the total size of the locus is considerable. Also, rare intra-locus and intra-gene recombination events are assumed to have contributed to the high level of B hordein polymorphism present within and between genotypes.

We recently identified an intra-locus Hor 2 recombinant in the cross Bomi × P 12/3 (Shewry et al., 1987, 1988). The endosperm halves of 281 F₂ seeds were analyzed by SDS-PAGE for alleles at Hor 2 and Hor 4 (Hrd G). One seed appeared to be a Hor 2 recombinant and the embryo part was grown, selfed and reselected in the F3 generation to give a series of homozygous F₄ lines.

SDS-PAGE of one of these lines shows that the characteristic single high M_r B3 hordein band of P12/3 is combined with the low M_r B1 hordein doublet of Bomi (Fig. 1, track d). The single band encoded by the Hor 4 locus of P12/3 (arrowed in track b) is absent. This has been confirmed by more detailed 2-D analyses. Molecular analyses are in progress to determine the number of genes that have been exchanged, and whether they include members of one or both of the major subfamilies. Fig. 1.

References:

Brandt, A., Montembault, A., Cameron-Mills, V. and Rasmussen, S. K. 1987. Primary structure of a B1 hordein gene from barley. Carlsberg Res. Commun. 50:333-345.

Bunce, N. A. C., Forde, B. G., Kreis, M. and Shewry, P. R. 1986. DNA restriction fragment length polymorphism at hordein loci: application to identifying and fingerprinting barley cultivars. Seed Sci. Technol. 14:419-429.

Faulks, A. J., Shewry, P. R., and Miflin, B. J. 1981. The polymorphism and structural homology of storage polypepties (hordein) coded by the Hor 2 locus in barley (Hordeum vulgare L.) Biochem. Genet. 19:841- 858.

Forde, B. G., Heyworth, A., Pywell, J. and Kreis, M. 1985. Nucleotide sequence of a B1 hordein gene and the identification of possible upstream regulatory elements in endosperm storage protein genes from barley, wheat and maize. Nucl. Acids Res. 13:7327-7339.

Kreis, M., Rahman, S., and Miflin, B. J. Forde, B. G., Pywell, J., Shewry, P. 1983. Sub-families of hordein mRNA encoded at the Hor 2 locus of barley. Mol. Gen. Genet. 191:194-200.

Shewry, P. R., Parmar, S., Franklin, J. and White, R. 1988. Mapping and biochemical analysis of Hor 4 (Hord G), a second locus encoding B hordein seed proteins in barley (Hordeum vulgare L.). Genet. Res. In press.

Shewry, P. R., Parmar, S., White, R. P. and Franklin, J. 1987. Mapping and biochemical analysis of the Hor 4 (Hrd G) locus encoding B hordein-like polypeptides.BGN 17:87-89.