cDNA clones for mouse parotid proline‐rich proteins

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Abstract
CDNA clones for mRNA sequences regulated by isoprenaline in mouse parotid glands were identified by differential colony hybridisation and all hybridised to a diagnostic proline‐rich protein (PRP) oligonucleotide. They were divided into two cross‐hybridisation groups, A and B, which were shown by hybrid‐selected translations to encode acidic PRP and basic PRP, respectively. The A‐type subgroup consisted of sequences homologous to the previously identified mouse PRP genes MP2 and MP3. The B‐type subgroup comprised clones for the previously identified cDNA pUMP125 (MP4) as well as other PRP sequences. Six of the B‐type clones contained a novel PRP cDNA (MP5) and these were sequenced. The composite MP5 cDNA was 897 nucleotides long and contained an open reading frame capable of encoding a 260‐residue‐long salivary PRP precursor (30% Pro, 19% Gln and 18% Gly), containing nine variant repeat units of consensus PGNQQGPPPQGGPQQ(GPP)R(PPQ). MP5 was 80% identical to the sequence of MP4 and had a high degree of similarity (60%) at its 3′‐untranslated region to rat salivary glutamate/glutamine‐rich protein (GRP) cDNA. Two MP5 clones contained a 273‐bp intron‐like insertion in the 3′ untranslated region, being derived, therefore, from incompletely spliced MP5 transcripts. Northern blotting showed that, although PRP mRNA species were induced by isoprenaline, a B‐type PRP mRNA was present in normal parotid glands. RNA dot‐blots probed with PRP‐genespecific oligonucleotides established that MP3, MP4 and MP5 PRP mRNA were all induced by isoprenaline.