The Florida State University
The Single myb histone (Smh) gene family of maize; detection of two PCR products from maize with primers for Smh1
--Figueroa, DM, Bass, HW
The Single myb histone (Smh) genes of maize make up a small gene family that was discovered as part of a screening for telomeric protein genes (Marian et al., Plant Physiol 133:1336–1350, 2003). The SMH proteins may be unique to higher plants. We have classified them as having a unique triple-motif protein organization an N-terminal single MYB-like/SANT domain, a centrally located motif homologous to the globular domain of the linker histone (H1/H5), and a near-C-terminal coiled-coil domain. The presence and arrangement of this triple-motif are diagnostic for SMH proteins. GenBank accession numbers and ChromDB synonyms for the maize Smh genes are summarized in Table 1. We found a similar Smh gene family in Arabidopsis (SMH gene family page at TAIR).
In addition to the five genes for which full-length cDNAs had been obtained, we found evidence for a sixth Smh gene by PCR analysis with primers for Smh1. The six Smh loci were numbered in sequential pairs to reflect our idea that they represent three closely related pairs of genes Smh1/2, Smh3/4, and Smh5/6 as described by Marian et al. (2003). The pairs are presumed to be duplicate but unlinked. Smh1/2 and Smh5/6 are phylogenetically more closely related to each other than to Smh3/4. Smh3/4 show the most frequent hits in BLAST searches of ESTs.
PCR primers designed to detect Smh1 but not Smh3-6 amplified two products using inbred lines B73 or Mo17 (see Fig. 1A & GenBank sequences AY328854 for B73 Smh1-STS and AY328855 for B73 Smh2-STS). The two bands of the STS doublet segregated independently in the IBM recombinant inbred lines and were mapped by Marian et al. (2003) using the IBM DNA mapping kit. The map position of Smh2-STS allowed for positional cloning of the Smh2 gene.
The lack of a cDNA or EST corresponding to the Smh2 locus initially indicated that the Smh2-STS might be from a non-functional gene or a rarely expressed gene. We checked a number of different maize lines to see how often the SMH1-57710/11 primer pair would produce the PCR product doublet. As shown in the Figure 1A-C, we found that all of the 11 lines of Zea sp. tested produced two abundant PCR products (the lines are B73, Mo17, CM37, T232, CO159, TX303, W23, Knobless Tama Flint, Knobless Wilbur’s Flint, Zea diploperennis, and Tom Thumb pop corn). In contrast, DNA from Tripsacum dactyloides produced a single band, whereas oat (Fig. 1C, Avena sativa) DNA and a no-template control reaction (Fig. 1C, H2O) did not produce detectable PCR products. The single band from Tripsacum was amplified from leaf DNA isolated from three different sources of T. dactyloides (2×-diploid and 4×-tetraploid from M. Eubanks, and EGG-Eastern gamma grass from USDA). Further analysis of the Tripsacum-derived PCR products may help clarify whether Tripsacum has one or both members of the Smh1/2 pair. A recent EST for a partial Smh2 cDNA has been observed (see ChromDB SMH102/hon112), indicating that the Smh2 locus may house a functional gene.
Table 1. Maize Smh genes.