Characterization, chromosomal mapping, and expression of different ubiquitin fusion protein genes in tissues from control and heat-shocked maize seedlings.

Organisms possess at least two multigene families of ubiquitins: the polyubiquitins, with few to several repeat units, which encode a ubiquitin monomer, and the ubiquitin fusion (or extension) protein genes, which encode a single ubiquitin monomer and a specific protein. This report provides details about two ubiquitin fusion protein genes in maize referred to as MubG7 (uwo 1) and MubG10 (uwo 2). Each has one nearly identical ubiquitin coding unit fused without an intervening nucleotide to an unrelated, 237-nucleotide sequence that encodes for a 79 amino acid protein. The derived amino acid sequences of the two fusion proteins show that they differ by five amino acids (substitution by either a serine or threonine). MubG7 maps to chromosome 8L162 and MubG10 maps to chromosome 1L131. Analyses of the role(s) of these genes in response to heat shock (1 h at 42.5 degrees C) reveal that the level of these fusion protein mRNAs in the radicles or plumules from 2-day-old seedlings does not change; however, heat shock does cause a marked reduction in the accumulation of these same gene-specific mRNAs in the radicles and plumules of 5-day-old seedlings. These data confirm the suggestion from our earlier work that there is precise modulation, in a gene-specific manner, of the response to developmental as well as environmental signals.

Characterization, chromosomal mapping, and expression of different polyubiquitin genes in tissues from control and heat-shocked maize seedlings.

Polyubiquitin transcripts accumulate in plant and animal cells following a heat shock. Most species have a few to several polyubiquitin genes; within a species, the genes may differ in nucleotide (nt) sequence and (or) the number of 228-nt repeats encoding the ubiquitin monomer. This study examines three maize (inbred Oh43) polyubiquitin genes. Two of the genes, MubG9 and MubG5, possess five repeats; the third, MubG1 possesses seven repeats. Sequence analyses of the genomic clones, MubG9 and MubG1 and a cDNA clone, MubG5, reveal that they differ primarily from each other in their nt sequences 5' and 3' to their open reading frames. MubG1 contains a 1004-base pair (bp) intron in its 5' untranslated region. Using gene-specific probes, we show that the amount of polyribosome-associated mRNA transcripts from MubG9 isolated from 2- and 5-day old plumules and radicles is unchanged by heat shock. While the amount of transcript from MubG1 and MubG5 on the polyribosomes in plumules and radicles of 2-day-old seedlings is also unchanged by heat shock, the levels of these transcripts are elevated considerably in similar tissues from heat-shocked 5-day-old seedlings. Similar or identical gene-specific probes were employed to map the genes using the recombinant inbred method. MubG9 maps to chromosome 4L position 186; MubG1 maps to 5L104 and MubG5 to 4L188. The opportunity to use gene-specific probes extends the evidence for distinct modulation (time and tissue) of polyubiquitin gene expression in maize and provides the basis for locus assignment within the genome.