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.