A cloned maize genomic sequence shows homology to meiosis-specific cDNA clones from Lilium and to a small heat-shock protein gene of Glycine max

Despite the central role of meiosis in the genetics of all higher organisms, relatively little is known about the gene activities which accompany this basic process. For several years I have been working with a cDNA library prepared from Lilium microsporocytes, which at present provide the only source of plant material biochemically amenable to this type of recombinant DNA technology (R. Appels, R.A. Bouchard, and H. Stern, Chromosoma 85:591602, 1982). These studies have shown that a particular set of cDNA clones representing a group of transcripts selected because they were expressed abundantly and specifically during meiotic prophase and showed homology to genomic DNA of wheat, rye, and maize, originate from a complex set of repeated sequences termed the EMPR (Expressed Meiotic Prophase Repeat) sequences. Studies of this set of sequences have now been extended directly to maize. Screening of a maize phage-lambda genomic DNA library, after initial difficulties (see accompanying report), has yielded a number of clones containing genuinely homologous sequences for the EMPR cDNA clones. A plasmid subclone for one of these regions, pZm9'-8, has been extensively characterized, and shows a number of characteristics which suggest that functionally significant aspects of the EMPR sequences are conserved in maize. The portion of its insert showing readily detectable cross homology to lily EMPR clone probes has been found to correspond to the region of these cDNA's which shows the strongest sequence conservation among different repeat subfamilies within the Lilium genome itself. Hybridization of probe for this conserved segment of pZm9'-8 to poly(A) RNA from successive stages of lily meiotic tissue shows that it is homologous to transcripts abundant only during meiotic prophase, just as is seen for the lily-derived EMPR cDNA clones. While preparation in quantity of pure meiotic prophase cells is not feasible with maize, it can be shown that transcripts homologous to pZm9'-8 are specifically present in RNA prepared from spikelets containing meiotic prophase microsporocytes, and absent from somatic tissue under standard growth conditions.

Recently, the inferred amino acid sequence of the conserved portions of the lily EMPR subfamilies has been found to be clearly homologous to the sequence predicted for a small heat-shock protein from a cloned soybean gene (Czarnecka. et al., PNAS 82:3726, 1985). In light of this observation, I have compared the inferred sequence of a section containing the most conserved portion of one of the lily EMPR clones and the corresponding segment of the published soybean clone sequence with the sequence obtained for an M13 subclone, from pZm9'-8 containing the most strongly cross-reassociating portion. As shown in the accompanying figure, the inferred amino acid sequence for at least this section of the maize clone is clearly homologous both to the lily EMPR and the soybean hsp-gene products. The fact that such substantial homology with both the conserved portion of the EMPR and the soybean clone lies here is particularly significant in light of the fact that this region corresponds to the portion of the small hsp heat shock genes where the strongest homology among different genes both between genomes and within the same genome has been found to lie. It therefore appears at the very least that an important functional domain is shared by small hsp heat shock genes, a large repetitive family naturally expressed during meiosis in lily, and a set of maize genes identified through their homology to the latter. Additional sequencing studies to examine further the nature and extent of this homology over the rest of the pZm9'-8 and other maize EMPR cognates, and other experiments examining whether transcripts complementary to maize EMPR genes are induced in somatic tissue during thermal stress, are underway.

Figure 1. Alignment of inferred amino acid sequences for conserved TaqI fragment of Lilium EMPR cDNA clone pLEc6, Glycine genomic clone gmhsp 17.5-E, and Zea genomic subclone pZm9'-8. Dotted lines indicate amino acid residues homologous between the lily and maize sequences, while solid lines indicate residues common to all three, lily and soybean (upper four of six lines in each row), or soybean and maize (lower four of six lines).

Robert A. Bouchard
 
 


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