To provide tools for understanding collagenase gene expression in rheumatoid arthritis, we have isolated and characterized genomic clones for rabbit synovial cell collagenase. These clones represent 2 types of collagenase gene, at least 1 of which is transcribed in synovial fibroblasts. By examining the rabbit genome in situ, we provide evidence that there are only 2 different synovial cell collagenase genes found in a haploid genome. Amplification of these genes is not a mechanism far collagenase messenger RNA induction by phorbol esters.The metalloproteinase, collagenase, plays a primary role in the joint destruction seen in rheumatoid arthritis (1). Vast excesses of this enzyme are secreted by the fibroblasts of the rheumatoid pannus. The fact that collagen breakdown can be initiated only by collagenase makes this enzyme rate-limiting in collagenolysis. One approach to the successful control of collagenolysis in diseases such as rheumatoid arthritis may lie in inhibiting the production of collagenase. Thus, an understanding of mechanisms regulating Studies in our laboratory, designed to understand the control of collagenase secretion, have been facilitated by the development of a cell culture system of synovial fibroblasts from rabbits (2). These cultures secrete negligible levels of collagenase when untreated. However, they can be stimulated to secrete major amounts of collagenase by treatment with numerous agents, including the tumor promoter, phorbol myristate acetate (PMA) (3), and crystals of monosodium urate monohydrate (4). Furthermore, additional agents such as retinoic acid or dexamethasone can reverse or suppress induction (5). To date, experiments with human synovial cells have corroborated our findings with the rabbit cell studies, and have thus validated the rabbit system as a model (6,7).Earlier studies demonstrated that inducers act to increase the synthesis of new collagenase protein rather than stimulate the release of previously synthesized enzyme stored intracellularly (8). With the development of a complementary DNA (cDNA) clone for collagenase, we showed that on induction, an increase in immunoreactive collagenase secreted into the culture medium is directly correlated with increased collagenase messenger RNA (mRNA) levels (9). Several steps in RNA metabolism could contribute to the increase in collagenase message levels, including increased transcription from the collagenase gene(s), greater mRNA stability, or changes in processing of pre-mRNA to mRNA. To study the contribution of these processes to mRNA induction, as well as the mechanisms by which they operate, requires more than a cDNA clone, which is merely a copy of mRNA. The collagenase gene itself must be isolated