Serratia marcescens, a Gram-negative bacterium which causes disease in plants and wide range of both invertebrate and vertebrate hosts (Grimond and Grimont, 1978), is an opportunistic human pathogen (Kurz et al., 2003). Various clinical isolates of Serratia marcescens harbors plasmids that code for antibiotic and heavy metal resistance (Hedges et al., 1975, 1977; Stanisch and Ortiz, 1976; Taylor and Summers, 1979; Griffin et al., 1987). Due to its clinical and industrial relevance, these isolates are of great scientific interest. At present, the broad host range vectors obtained from different organisms is used for genetic manipulation of S. marcescens (Rangwala et al., 1991; Sakurai et al., 1993). Even though a large number of native plasmids have been reported from Serratia marcescens, they are of huge size (Taylor and Summers, 1979; Hedges et al., 1975, 1977; Stanisch et al., 1976; Griffin et al.,1987 ) and only one of which is fully sequenced (Gilmour et al., 2004 ). Endogenous plasmids that are relatively small in size is essential and would be an added advantage in effective driving of gene expression in Serratia marcescens and could facilitate construction of shuttle vectors for genetic analysis of Serratia marcescens.

When compared to the all other plasmids reported so far from Serratia marcescens in sizes of over 70kb, this plasmid is only 4241 bp long with 53% G+C content and has five coding sequences representing a coding percentage of 65.41. This small plasmid consists of one Tdh gene, four mobilization genes, mobCABD, and an origin of replication homologous to those of ColE1-type plasmids. Analysis of the five open reading frames identified on the plasmid suggests the presence of genes involved in replication and mobilization containing sequences homologous to the bom region and mobCABD genes of ColE1 and Tdh from Acinetobacter baumannii str. AYE. Results also indicate that pRK10 does not encode any gene for antibiotic /heavy metal resistance. This relatively small plasmid will be useful for construction of shuttle vectors to facilitate the genetic analysis.