Regulation of ribosomal protein genes has been studied in Escherichia coli, yeast, animals and to a lesser degree in plants. Mutations in ribosomal proteins have been shown to be responsible for several different phenotypes, including the minute phenotype in Drosophila, and trichodermin resistance in yeast. Resistance to trichodermin in yeast is conferred by a single amino acid substitution in the L3 protein. In Arabidopsis, the L3 homolog exists as a three gene family, which we have named Arabidopsis Ribosomal Protein (Arp) 1, Arp2 and Arp3. Arp1 and Arp2 appear to be fully functional, however, Arp3 is a processed pseudogene. The existence of more than one functional member of a ribosomal protein gene family is unusual for prokaryotes and animals, but is apparently quite common in plants, with most gene families having two or three members. However, Arp1 and Arp2 are unique in their level of divergence. In other ribosomal protein gene families with multiple members, there is very high conservation at the amino acid level, with figures of 99% conserved not unusual. The amino acids of Arp1 and Arp2 are 85% conserved. This provides the opportunity to determine the distribution of each of the gene products at the protein level, a task not possible with other ribosomal protein genes.

Previous studies using Northern analysis and reverse transcription-polymerase chain reaction (RT-PCR) have shown Arp1 to be transcribed at a high level and is found primarily in the poly(A)+ fraction after separation on an oligo dT column. Arp2 was shown to be transcribed at lower levels and is found primarily in the poly(A)- fraction by Northern analysis and RT-PCR of oligo dT separated RNAs. Despite its lower level of transcription, transcripts are quite abundant in root tissue. Arp2 transcripts can be isolated from polysomes and amplified using an oligo dT primer for the RT primer, indicating that a fraction of the transcripts may have a polyA tail. Current studies are focusing on detecting Arp1 and Arp2 proteins using antibodies specific for each. Also, the 5’ flanking and 3’ flanking regions of each gene have been cloned and will used in GUS constructs to better analyze the expression patterns of each gene. The potential of a specialized function of Arp2 in roots is currently being pursued.