Molecular basis for allelic polymorphism of the maize Globulin-1 gene

An abundant protein in maize (Zea mays L.) embryos is a storage globulin encoded by the polymorphic Glb1 gene. Several Glb1 protein size alleles and a null allele have been described. Here we report the isolation and nucleotide sequence analysis of genomic clones corresponding to two Glb1 size alleles (Glb1-L and Glb1-S) and to the Glb1-0 null allele. The Glb1-L and Glb1-0 alleles differ from Glb1-S by the presence of small nucleotide insertions which are imperfect or perfect duplications, respectively, of adjacent sequences. In the case of Glb1-L, the insertion is in-frame and results in a protein larger than that encoded by Glb1-S, whereas in Glb1-0 the insertion causes a translational frameshift which introduces a premature termination codon. Although steady-state levels of Glb1-0 transcripts are extremely low in Glb1-0/0 embryos, nuclear transcription assays indicate that the Glb1-0 gene is transcribed at a level comparable to that of Glb1-L. This suggests that the low amounts of Glb1-0 transcripts in the cytoplasm may be due to mRNA instability.