Code Proteins of Gene Family - 

A number of plant and animal protein coding genes are also found in. multigene families. However, these families are much smaller than the rRNA gene families and it is much more common for protein-coding genes to be individual units, even when they are located fairly closely together. A good example of clustered individual units is provided by the genes for the small subunit of RuBP carboxylase (rbcS genes). These genes have been extensively studied in several plant species, including petunia, pea, and duckweed (Lemna gibba).

In all three cases there were multiple copies, Lemna had about thirteen copies, pea six, and petunia eight. Although the genes in anyone plant are highly homologous within their protein-coding regions, they differ from one another in transcribed but untranslated regions located at the 5' - or 3' ends of their coding sequences. By making gene specific probes from the 5'- or 3' -regions or by sequenc­ing cDNA clones and comparing their sequences to those of the genes, it is possible to show that most of the genes are actually transcribed. Therefore, the multiplicity of gene copies does not merely reflect the presence of inactive copies or "pseudogenes" (partial or defective copies), although the presence of some pseudogenes cannot be ruled out

Multigene families can typically be divided into subfamilies on the basis of degree of homology between different groups of family members. Perhaps the best studied example of this phenomenon is the zein multigene family. Zein proteins can be divided into two major classes on the basis of their molecular weights. The DNA sequence of these two classes shows considerable homology, indicating that both classes probably evolved from the same ancestral gene and can be regarded as belonging to one large gene family.

This large family can be divided into the two groups just men­tioned and one of the groups further subdivided. From an analysis of Southern blots it appears there may be at least 40-50 genes in an inbred line of maize. Although not all these genes may be functional, the isolation of cDNA clones containing at least ten different sequences indicates that considerable heterogeneity exists even among genes that are expressed